I have an older C class where the house battery in under the hood. When camping and I leave the stereo running in the cab will that drain both battery's or just the car battery? I assume it's wired in like a normal car and run;s only off the main car battery not the house battery. Instead of running the main genny I was thinking of bringing my little Honda 1000 and run it to crank some tunes all night. With teh genny plugged in will that keep both batteries charged or just the house battery. Don't want to be starting the coach every 45 min to charge the main battery.

While the stereo is on, disconnect the chassis battery, if it stays on then you know the house batt is tied in. Before you plug the coach into the porta gen, check the voltages at the batts, (after they've had time to rest from any recent driving or charging, a couple hours is fine) then fire up the porta gen and check the voltages, that'll tell you whats going on with the batts on shore power.

I assume since the house battery charges while driving that they're tied in.

Typically they're isolated on the draw side so yes, the house batts will get a charge will driving but the chassis batt won't drain while camping which could strand you.

On my class A, The stereo is on the house battery only.