This is a great question, and the answer depends on the exact equipment your van has. In a nutshell, however (in the interest of clarity I am going to gloss over some technical details; if they become important, we can deal with them later):
- When you plug your van in, that brings in ordinary home electricity--this is known as AC, or alternating current, and it is at 120 volts.
- Your house batteries run on DC, or direct current, at 12 volts.
- It is not possible to directly connect 120v AC power to a 12v DC battery. The power needs to be converted from 120v AC to 12v DC.
- The other thing about your batteries is that they need to be charged in a very specific manner in order to maximize their life. Most house batteries (either lead-acid or AGM type) need to go through several distinct charge phases:
1. Bulk Charge--from 0% up to about 80% charge, the bulk charge is all about cramming as much charge into the battery as quickly as possible.
2. Absorption Charge--from 80% to 100%, the object is to slowly fill in this last bit, because the battery is unable to maintain the high charge rate of the bulk charge as it gets fuller.
3. Float Charge--once the battery is full, the goal is to trickle in just enough current to keep the battery full.
- Another detail: Batteries are a bit finicky. In order to maximize their life, you need to never discharge them below 50% of capacity, and when you charge them, you should charge them to full and keep them that way.
- And a final detail: if you have lead-acid batteries, they will have filler caps, and they need to be checked periodically to be sure that they have enough fluid in the battery cells.
So--what this means is that in order to maximize the life of your batteries:
1. When you are using them, do not discharge them below 50%;
2. When you are plugged in, you need to convert the 120v AC power to 12v DC power, and you need to control the power going to the batteries so that they can go through the three charge phases, get filled to the top, and kept that way.
Since you have a 120v AC plug in, this means that your van has some kind of converter or battery charger that is converting the 12v AC power to 12v DC before it goes into the batteries.
The older style converter/chargers do NOT take the batteries through the three stages I discussed above. They just do the bulk charge. If you keep a battery plugged into a bulk charge beyond the 80% level, you will greatly shorten the life of the battery.
If you have a modern three-stage charger, then keeping the batteries plugged in will extend their lives.
1. Make a habit of not discharging your batteries below 50%.
2. Find out if you have a single-stage converter/charger, or a three-stage converter/charger. If you have a single-stage unit, then you will not have a way to charge your batteries in a way that extends their life, and keeping your rig plugged in will definitely shorten the life of the batteries.
3. If you have lead-acid batteries, you will need to periodically check the fluid levels in the cells. Be careful with this; the fluid is an acid, and it is highly caustic.
1. Identify what type of converter/charger you have. If you want some help with that, post some photos here, and the gang will help you figure it out.
2. Identify what kind of batteries you have--lead-acid or AGM. Post photos if you want help.
Once you have this information, it will be possible to come up with some more specific recommendations for you.