I can't say what exactly is wrong in this instance, but it sounds like overall you are sort of running two types of systems. What I mean is that you are "working" your house bank as if you had a "good" house system; but you are really set up in a more simple way, and with that type of set-up you really only want to use your house batteries very minimally (more like "the old days"). When I say the old days, I mean you would either be plugged in at a campground, or you would be using lanterns and hand pumps.
Not to sound too dramatic, but now it's like you're running long distance without a fuel gauge or an odometer, and you only start with a quarter tank of gas. Something is bound to go wrong!
Not that you are doing so on purpose, but it happens a lot due to modern usage patterns combined with system setups that are basically the same as they were 20+ years ago (in fact, your setup sounds similar to the way my 1997 SMB came, and in that rig I had to either drive, plug in, or use things VERY sparingly on an overnight stop (now a friend owns that rig and we have set it up for boondocking/modern usage, but that's no the way it came or was set up when I bought it).
A "modern" setup does need some money thrown at it, and probably some time --- unless you are lucky enough to have a good electrician nearby. The good thing is that once you've bought the components/wire/etc. it's not hard to learn the basics and do it yourself, so in most cases you don't need to pay for labor.
One thing is that whatever you use, you have to be able to put back. So say you decide you want to be able to use 100 amp hours per day, and you want to draw your batteries down to 50% (on average) for good lifespan, so you get 200 amp hours worth of batteries and put them in. Day one is great, you are perfectly set up. But if there is going to be a day two, three, or four, then you have to have a way to put back the amp hours as well. This typically means solar panels, generator, or plug in (the last two requiring a good smart charger, IMO). On my boat (as it is set up now), I only have the wimpy engine alternator for charging, so that made me limit the battery bank size to 100 amp hours, of which I can use 50 before I have to get underway for a loooong day in order to put the amps back into the battery. So in that case if I had 300 amp hours of battery I'd NEVER get caught back up on a trip (not to mention it is a "dumb charger" and so it's hard to ever get the last 15% of charge in; this is why I'm adding solar).
So, one way to go about it is to figure out your usage (amp hours per day x number of days between charging) and then set up for that. Or, you can figure out what your limiting factor is, and just build a system to that, and know you've done the best you can -- but then you limit usage accordingly.
So, for example, if you can only fit one house battery of 100 amp hours (Group 27, say), then you know you can only use (around, there is no magic line) 50 amp hours, and then you need to be able to put back 50 amp hours, preferably in a "smart" way. If you can fit two batteries, then you can double that, but you also have to double what you put back (but then maybe you can do it at the end of a weekend).
With something like a huge Class A, you can usually fit tons of batteries and/or solar, so you could easily built an overly huge system, but that's not likely possible on an SMB.
So... what is your usage pattern? If you are trying to keep to a budget, and you only go for weekends (and your usage is "reasonable," you could put in a larger house bank and a smart 110 charger, and then just plug in before you leave home and right when you get back (with reasonable usage and enough battery power). If you are out for longer, then you'll likely need some other way to put power back, such as a generator or solar panels (I hate listening to generators, so I'd vote solar panels).
And then the gas gauge, a battery monitor. Normally that's the first thing I recommend, but if your budget is tight, and you can precisely calculate your usage over a short period of time being out (like one weekend), then I might say you could just go ahead and put your immediate money into batteries and a charger. Although a monitor is always nice. For just one "fit and forget, just tell me the SOC," I like the Balmar Smart gauge. For more information (but some input needed, especially as batteries age), then something like a Victron BMV 700S, which is an amp counter type meter. Or if you are a total nerd (
), both. There is also the school of thought to just put in cheap batteries and replace them when they go bad, which can work in certain situations.
Here is an example of a system we put on my buddy's SMB. Note that we did 300 watts of solar because he lives in a cloudy area, and there was no problem fitting that much (and solar panels are relatively cheap now); but the system would probably have worked fine with 200 watts, especially in a sunny climate. His usage is to want to be able to stay out indefinitely without plugging in, to run the (for now, old inefficient Norcold) refrigerator; to listen to the radio all day; to charge laptop, phone, etc.; and to run lights, most of which are not currently LED. He does not have a smart charger yet (110).
1) Three Renogy aluminum/glass 100 watt panels on PH top, with home-designed aluminum brackets secured to the roof with VHB tape.
2) Cable Clam for bringing #10 wire from solar panels into rig (very short run).
3) #4 wire for run from PH "rim" level to solar controller.
4) Blue Sea on/off switch between panels and controller.
5) Blue Sky Solar Boost 2512iX-HV MPPT charge controller, with optional display that also has battery monitor functions (includes shunt).
6) Two Lifeline Group 31 GPLXT AGM batteries
7) In addition, we used mostly Blue Sea components to upgrade the overall electrical system, which was not properly fused, etc.
I already had a big crimper tool, plus quite a bit of cabling and lugs on hand, so only the #4 cable had to be purchased in the wiring department. That helped. I'd also say that he could have gone with Group 27 batteries to save a bit of money if he'd wanted to (but with only room for two batteries, he decided to get the 31 XT's because they are 125 amp hours each).
If he were to make next steps, they would be as follows:
1) Upgrade tiny wire from engine compartment to house bank (brings alternator power back, has huge voltage drop and hard to fuse as it is so small) (For some reason he stubbornly did not want to do this at the time, but we all have our quirks
)
2) Smart 110 charger.
3) Newer refrigerator (Truck Fridge or the like).
4) LED lighting.
Okay, by now you're probably thinking "Whoah, I just wanted to keep from killing my one house battery!"
I did live like that traveling in my SMB for months at a time (one house battery, no real means to recharge except driving (with tiny wire/voltage drop) or plugging in (stupid charger) and with no monitor. I made it work, but I had to live "old style" when parked and not plugged in (flashlights, turn off fridge, use a big sleeping bag vs. the furnace) -- and then I charged things directly from the dashboard when driving (computer, phone). I had to keep a move on.
Now of course my buddy has the system I always wanted on the SMB
I hope some of this is helpful, and if you do want to improve the system somewhat, maybe post your usage pattern (i.e. how many days out in a row, how do you expect to "live," etc.) for more tailored ideas.