More generator vs. AUX-START

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.