Like BigBlockTank says, check the battery first. Most auto parts places will load test it for free. Take it in for the test.
Another thing to check if the battery checks OK... My ground cable uses a clamp type terminal ( like
THIS ). There was some corrosion inside the clamp that was preventing the coach from getting a good ground when cranking. I cut off the corroded cable end and put a new clamp on and have had no problems since. Everything seemed to work til I tried to start and the high load was too much for the damaged ground cable.