I would double check all battery (chassis) connections and then follow both the positive and negative cables to the connections with chassis ground and positive distribution point.
I had similar problem yesterday with my 06 E450 chassis. I went to start the engine, 1st time after winter. The MH is plugged in to shore power all the time and a battery tender on the chassis battery, water level is good in all batteries.
The engine would not turn over on the Ford battery or with the Boost switch or with a heavy duty battery charger/boost. I was ready to invest in a new chassis battery (the one in the MH is OEM, 13 years old). One last effort, cleaned and tightened chassis battery cables and vroom, Still started today.
Ken
2006 Winnebago Outlook 29B E-450.
2012 Honda CR-V AWD
Blue Ox Aventa LX tow bar and Brake Buddy Vantage.