I'm thinking unless you've verified this is in fact the cap being an issue, the shut down is being caused by something else.
I'd first look at the gen set engine oil level.
Be sure you have enough fuel to support running the gen set, don't know if that will effect your coach as it does mine.
Be sure you have a full radiator, you may have a coolant level shut down switch also.
These are just guesses on my part, other than an electrical out put problem I'm lost. Hope you find your fix with one.
Dan & Loretta, US Army Retired Aero Scout Pilot
2012 Fleetwood Providence 42P Class A/DP
Spartan Chassis, 8.9L Cummins 450HP