http://www.leisurevans.com/owners/manuals/2013_Unity.pdf.
Quick read...
The "battery voltage" on the house control panel only reads voltage on house battery. Not starter battery.
Is the radio switch off? If left ON, will drain the chassis (starter) battery even if radios are off. You could have minimal power to boot up but not enough to close the starter selonoid.
Inverter/converter does not charge either battery set.
Recommendations agains fast charging batteries!
Engine alternator charges both battery sets....
Shore power charges both battery sets.....
Charging fuse identified under driver seat.
Do you have a booster switch to apply coach battery power to the chassis battery to jump or boost a dead starter battery? Not shown in manual.
FWIW... I'd tach up the generator and turn on the coach LP heater and warm the thing up. Manual's not clear whether or not the generator will charge both battery sets... It does charge the house set.... You could get lucky! Warming up the cabin may put something right and let it start.
If you have a small electric heater and a heavy extension cord... Put the electric heater under the hood and power it up with the generator. Keep an eye on it Don't melt or burn anything.
If you have a jumper pack, use the under hood jumping connections. Red post on the air cleaner... Black on fender.