Dual batteries in an older Lance?

Somehow, time marches on and I now have an older Lance. When we bought it, I was in your shoes and measured and hoped I could squeeze two batts. in a box made for one. But, alas, it was not to be. So I ganged all three (exactly the same battery) with a #4 wire coming from the engine compartment to the camper plug. Maybe I'm living on borrowed time, but we've had very good luck with battery power. Only once, when I left the DC mode on the fridge over night did the power consumption (which is mighty on the fridge) take me down so the truck would not start. You only make that mistake once. We are now battery misers, using propane on all appliances unless we're traveling more than 50 mph, in which case the propane flame blows out necessitating a return to DC power. We use a 4-D-cell lantern when we have our nightly backgammon games. It lasts for weeks with no power drain on the truck/camper batts. Minimal use of the incandescent bulbed interior lights. I know, I know, dump the incandescents. The only black cloud on the horizon is the increased use of personal electronic devices, some of which, like my 15" lap top can use a lot of battery power. It depends a lot on your traveling style too. We don't spend more than a night or two in one place with enough travel in between to recharge with the big alternator.
I wish there was an easy way with these older Lances.
jefe