Welcome from the Florida Panhandle
First off, welcome to the Forums. We're glad to have you with us.
Is your charging system set up to charge the engine battery along with the RV batteries? If not, the answer is obvious. If the engine battery was charging, the condition of the engine battery may have been poor going in. Did the engine battery take and hold a charge affter you found it depleted.
SuEllyn & Brian McCabe
WBCCI #3628 -- AIR #14872 -- TAC #FL-7
2015 FC 25' FB (Lucy) with HAHA
2005 Suburban 2500 Quadrasteer (Olivia) & 2011 Silverado 3500 (Fred) with Outfitter Truck Camper (Ethel)