This is easy as I just went through trouble shoothing exact same issue on my BT cruiser. There is a part that when you turn the key allows the rv alternator to charge the coach battery the runs all the acessories. When the key is off it also stops that accessorie battery from drawing from the starting battery. They make a battery isolator as its called. OR a dual battery relay with an isolator built into it as most likley you will find on your rv as I did with mine its more heavy duty.
Here is company link I am buying mine from
Inexpensive 12V 150+ amp smart DC battery isolator and smart split charge relay for RV, car, and truck applications
It also is setup to except the emergency battery start which most RVs have on them that if you coach battery is charged up and your enginge starting battery is dead this emergancy start switch allows the coach battery to open power when switch is held to enging battery and start engine. Like a jump start. Replace this and your charging issue will be corrected. Its very simple to replace.