I've installed fresh water tanks a few different ways on various boats. The typical way is with a deck fill, which is analagous to the outside side wall fill (so not what you want). But I have also had the fill inside, either with a screw on cap on the tank, or by using a deck fill but just putting it somewhere inside (counter top, wall with an angled fill, etc.). This works fine as long as you have your own hose (otherwise dragging dirty dock hoses through your interior is kind of gross) and/or a place where a drop or two of water won't hurt while filling. I've then used a hose with a "spray gun trigger" sort of end, so I can stop the water flow right when I want to.
If the cap is right on the tank, then you can't quite fill the tank as full, especially if you are on a slight slant, or rocking. If there is a neck on the tank, and/or a deck fill somewhere else inside (with a hose to the tank) then you can fill it all the way up (and even slightly into the neck/hose).
I have never had the vent incorporated into the fill. Instead, I have had a typical 3/8" or 1/2" vent fitting and run a hose from it. This can be run inside, and that can even be advantageous in terms of keeping airborne contaminants from being sucked into the tank constantly (when you draw water off). If it's a good bit higher then the fill, you shouldn't get water coming out.
I'm not sure how this last bit would apply to a pressure pump setup (as I had it on a foot pump setup) - there may be no difference but I'm just not sure - but in one setup I had the interior vent hose running up in a locker to around 4' above the tank. In that case the overflow (when filling - this was a deck fill so I just filled and filled until water overflowed vs. watching it) came out the sink faucet and then drained (overboard, as sink drains do on boats). That was handy.
Of course if you have no place BUT the interior vent hose or the fill for the "overflow" to go when filling, then you would want to watch it carefully (as I'm sure you would if filling indoors anyway) as it will burble up through the fill cap when it is full (presuming vent is much higher).
I like to put some sort of filter on the end of the vent. I think there would be "real" filters you could buy, but I have just have gauze fastened over the end of the vent hose. If you look at a typical vent hose that runs to the fill outside on an RV (terminates in little hole next to filler), you'll usually find it black with mold (gross!). It think this is mostly due to airborne contaminants being constantly sucked in (which of course likely get into your water tank as well). If it's exposed to light and is clear hose that could be a secondary reason.
Here are a couple of deck fills that you might consider using inside, if you have a place for one. These both take 1-1/2 hose; one note is that RV water tanks often take 1-1/4 or maybe 1-3/8" hose, so something to be aware of if you have that. These seal with an O-ring so will not leak even if water is right up to them. You use a little "deck key" to open and close them. They also have some with keyless caps, etc. but this is the classic time-tested style.
http://www.sea-dog.com/groups/1806-hose-deck-fill
http://www.sea-dog.com/groups/1810-angl ... -deck-fill
http://www.sea-dog.com/groups/1798-deck-fill-key
These are "gravity fills" so not "city water/pressure hose" type fills.
Then the vent fitting would typically be a 3/8" or 1/2" female threaded boss on the tank, into which you would put a male threaded adapter to either barb (for hose) or potentially PEX pipe fitting.
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