Turn off everything in the motor home. someone may have hooked up something new in the circuit for the starting battery instead of the inside system. then check for a voltage draw
Pull the fuses on the vehicle fuse panel(not the service fuse panel) and check across each fuse holder for voltage.
If you find nothing another thing to try is check by disconnecting all the small wires from the hot side of the starter relay and meter the wires one at a time till you find one with a load on it. some one may have added a accessory on the starting instead of the service battery system
Put a volt meter on the hot post and the other meter wire on the small wires one at a time and if one reads 12 volts the circuit is drawing a load.
My used RV had a number of these connections to the wrong service.
Once you have the problem cleared put a battery isolator between your service battery system and your starting battery system so that your starting battery is charged as needed by your service battery system charger.
Lynx/95 Amp battery isolator (08770) | Battery Isolator | AutoZone.com
I have a solar panel that keeps my service batteries charged and through the battery isolator it also keeps the starter battery charged when the RV in not in use. and not connected to shore power.