Check connections first.
Check to make sure your battery disconnect is working. I had mine fail ~3 years ago. Was able to start coach with boost switch. Bypassed the disconnect switch it and was able to start coach. The contacts must have been bad, they are internal in a sealed unit so couldn't really tell. Replaced it and that solved the problem.
Check battery with a hydrometer to see if any of the cells are are bad. If 1 cell is bad the battery is trash and will have to be replaced.
Last year I couldn't start my coach and found 1 cell dead, replaced battery.
2002 Monaco Windsor 38 PKD Cummins ISC 250 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi