No E-start. What?!

I work for an RV dealership, which just completed a four day show at the fair grounds. We had several Class C, on Mercedes Sprinter chassis, with a dead chassis battery. We've had this problem at previous shows. There is no Emergency Start button that we could find. The house battery BTW is fine.

Normally we jump the engine battery to start the diesel but today I tried using the generator and shore power. They didn't work. These were Thor Citations and the manual says the only way to charge the chassis battery is by driving. Is this info correct? I'd love to find a way to tie the generator or shore power to the chassis battery to get the motor running.