Check the solenoids that connect when a charging current is present, if so equipped. Most likely the issue. There are many variations of how RVs are wired to connect both battery systems when a charging current is available, I can't tell from here how yours are wired for this. Do you have a boost button on the dash for emergency starting? Does it still work? That would be step one to tracking down the problem.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI