adding more batteries

When I had a Ford chassis, there was plenty of room for more batteries behind the front grill but I never needed more than the 2, 6 volt batteries that it had.

We have a safari
And I put the extra batteries in the basement

Mick.....I've seen it done two ways. One....they mount them behind the grill on the frame. The only issue, you have to run a lot of wiring. Second, my neighbor just did the upgrade that you want to do. He did it on his Fleetwood Fiesta with the batteries mounted under the steps. He had a storage bay, directly behind the front door. He found a battery box that holds two six volt batteries end to end, not side to side. He mounted it against the wall between the bay and steps.

This installation was very clean, took up minimal storage and put the wiring within inches of the other batteries.

You didn't mention that your current batteries are acting up or that you plan to do a bunch of dry/boondock camping. If you are an RV park type of family, why upgrade now? Wait until they show signs of failure then up grade as suggested - T105s, T125s etc.

4 batteries under the step is a common upgrade on Winnebago gassers.

This post may help.

Get AGMs and you can put them in any position anywhere.

Yes, AH is everything on the batteries. Have you tried golf cart batteries wired in series parallel? They have A LOT more amp hours and typically are just a LITTLE bit taller than deep cycle.

You didn't say what the ampere hour rating is on the batteries. If they're typical 12v "deep cycle" marine/RV, then I think the number I saw floating around this forum was 62 AH. If that's the case, then just replacing those batteries with 6 volt deep cycle golf cart batteries such as Trojan T-105s will get you 225 AH, discharging to 50 percent basically doubles your capacity, so it would be like adding 2 batteries.

Another thing to consider; unless your existing batteries are new, you'll probably need to buy 4 so they would balance out.