charging of 2 camper batteries with truck alternator?

I'll call BS on camper heaters being oversized, it all depends on when and where you use your camper. On a November deer hunt, I've used both the camper's 12, 000 BTU furnace and a 1500 watt electric and still woke up to a very cold camper. We ran the genny all night and had heavy down sleeping bags.

I'm currently installing a 19,000 BTU furnace after the fan bearings packed it in ,on the original furnace. Maybe I can get some clothes drying, too ??