BIRD charging circuit
On my Newmar I have this charging circuit if you have it check the left side of left solenoid and you may have a 5 amp fuse plugged into holder mounted on left side with battery cables.
On mine the fuse plugs into a female spade clip with green wire within holder that some how worked loose from holder and popped out the top disconnecting from fuse, thus engine wouldn't start.
With my not so nimble fingers and a long longnose pliers held spade clip in holder and pushed fuse up into holder and wallah the engine would now start.

This 5 amp fuse also affected my battery cut off switch, would or wouldn't work at times.
Tried to turn the fuse holder toward the front but its mounted behind back nut against solenoid and I didn't want to loosen up internal parts of solenoid so left it as is.
I thought for sure it was going to be the starter solenoid because I traveling around this winter collected more salt from the roads it now looks like a seven year coach under the chassis, what a mess, have to clean all the locks to compartments and repaint undercoating.
Goes to show how much I new,easy fix once found.