I'm in the same dilemna with my 34', with two compartments, just to the inside of the rock guards on the outside, which happens to be just to the inside of the couch frame on the inside.
I wouldn't mind putting a couple of factory compartments behind the rock guards, but the problem is, the trailer is more curved there, so the factory ones wouldn' lay flush, and I really don't think there's room on the outside of the couch frame, especially if the curvature of the trailer makes them sit at an angle inside.
I've also thought of trying to relocate the converter to one side of the couch frame and trying to put two more compartments in the center. Haven't measured those either, so I don't think they'd fit, plus I'd be worried that the converter air flow would be too restricted.
It's really important not to let hydrogen build up in an area inside the trailer or two vehicle.
Finally, I've thought of putting a battery box on the tongue for the two extras behind the LP tanks, with a coupla hoses running down below the tongue for venting (keep corrosive gas away from things). Even with this, you'd want to pull the + lead for the converter off the first battery in the parallel setup and the - lead off the last... so it would be better to wire one internal battery to one external, the two externals together, and back to the other internal battery. By doing this, each battery is in series with the same number and length cables which parallel the batteries.
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where the first battery is in series with 2 cables... and the last in series with 8, you want:
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where each battery is in series with 5 cables, so the voltage drops are the same, even if the batteries going outside are longer:
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in this case each battery is in series with two longs and three shorts. (ignore the periods... I had to use them to make things line up)
Even then, when it's cold, the warm internal batteries are going to be losing capacity "charging" the colder external batteries, and will be contributing more of their capacity to loads. In this case, not even 4 new identical batteries are the same even with our best effort to cable them that way.
It's a danged shame that A/S builds 31-34' trailers with all the lights, etc (mine has 12 double tube flourescent lights) and installs two batteries. It's probably worse that some of the smaller models only have one battery compartment.
I've finally surrendered to the idea of trying to get by with two batteries, and use two of the strongest Group 27 I can find, the Trojan 27TMH
which would give very slightly better performance than a pair of T-105s, and much better performance being warm, than a pair of cold T-105s on the tongue.
I've also resigned myself to running them down to 20% charge. I don't plan on getting a set of batteries charged more than about 90% in a few hours/day of solar, generator charging, or driving, so that gives 70% (.7X230AH=161AH) of their capacity for use.
Four T-105 batteries on the tongue (besides adding more to the tongue weight) would let me run from 90% down to 50% while giving even more daily capacity (.4X450=180AH or two days at 90AH) and have a much longer life for the batteries.
But when it's colder and the capacity of the four T-105 batteries are down to 70% you'll have to also take them down below 50% to get more than 126AH.
The batteries not taken below 50% will last three times longer (i.e. have 3 times the discharge/charge cycles) than the batteries taken down to 20%. But because you're replacing half as many batteries when you do, this isn't 3 times the battery cost, it's 3/2 more.
Anyway, I hope you've gotten something out of all this rambling about all the thinkin' I've been doin' about the same battery situation.
I figure with 54/35/39 tanks, starting with about 3 gallons in the black tank and stoppin' with 2 gallons to go, and not being able to get the last 2-3 gallons out of the fresh tank, we've got 3-4 days of boondocking at 13-17 gallons per day with 70% of the fresh water winding up in the grey tank, and us contributing as much extra to the black tank as the water used for flushing.
If it's cold, our 34K btu furnace running 1/3 the time is going to use 1.25 40# tanks in 4 days (as well as 60AH/day), with most of the rest of the propane used for refrigerator, hot water heater, and cooking. And if it's warm, while they won't use any propane, a Fantastic Vent running on medium for 24 hours is also going to use about 60AH/day.
Given that we'll be using 110-120AH/day with fan or furnace, there's no way I can carry enough batteries to last 3, much less 4 days. And since you need one day plus two days backup battery capacity for solar without a generator, there's no point in us investing in solar for 3-4 days boondocking. If we had that much battery capacity, we'd just spend a night and recharge from electrical hookup wherever we dumped the tanks... not to mention get a chance to take a long, unlimited shower!
In the end, the answer for us is to have one day's usage battery capacity, and use a small generator every day. The real bonus is that if we run it from 6PM to 10PM, half of those daily AH (lights, TV, etc) never come out of the batteries, but directly out of the converter, so the batteries will be discharged to much less of a depth, and last longer. We also have the option of paralleling a 2nd small generator for those times we need air-conditioning, should we ever chose to boondock when it's that hot.
More ramblin'... hope it helped.