More generator vs. AUX-START

Ever since Onan RV generators quit having a built in battery charge back circuit, they have typically been wired to the house batteries and the converter keeps the batteries charged from the generator 120v ,you have a problem where the main heavy generator B+ cable connects with the house batteries, possibly at a terminal block, the wiring from the aux start relay between the two battery systems is solid and working and i (in your case) between the generator snd the house batteries so it by passes your connection problem as long as it is engaged
If you have the Battery Management system on your rv i would start there "Big black box with fuses and relays and lots of wires, both battery systems meet there, the aux start relay is there and the generator power cable , along eith the fuses and cables for leveler control, power seats all the goodies not part of the RV interior there are some DC breakers there for the house batteries, that might be tripped preventing the generator from having any 12v start & run power

Follow the generator cable from the gen set to the coach batteries.

You have two sets of batteries in your coach, chassis, and coach. When you push the Aux/start button, it ties the two sets together such that you are now using BOTH sets together to do what you want to do (i.e. starting the coach, or starting the genset.) The chassis battery starts the RV engine. The coach battery starts the genset. When you push the Aux/start button you add the chassis battery to the coach battery (in parallel). It is pretty obvious that coach battery is somehow not connected properly to the genset. Find that problem.