I just bought a Trail Lite B+ 211s with 8.1 Chevy engine. This is my fifth motorhome (others range from an old 20' Vagabond, and three Winnegagos, from 19 to 32'. This vehicle is fantastic. I am...
Well, you don't want to run both batteries down. This way you can run down the coach without affecting the chassis, or vice versa. Also, if one battery is way down and is being charged, the BIRD won't divert any charging to the other one.
I guess another aspect of it is that the chassis battery is a starting battery and the coach is deep cycle, and they have different charging requirements. You'd ruin the chassis battery if it was drawn down very many times during dry camping.