POWER PROBLEM WITH NEWMAR, NEWBIE

Help with new owner. Electical problem. I just got a 2003 Newmar Mountain Aire. 38 ft. gas. Chevy engine. Love it so far. Purchased 5 weeks ago and spent 3 weeks in backyard learning unit. Used generator 2 weeks with no problem. Kept it connected to normal 110 between times to keep batteries charged. 3rd weekend had 50 amp connection put in so we hooked to this and it was wonderful and quiet without generator. All working well. Gassed up and went to mountains this weekend with no issues. Had 50 amp hook up, no generator needed. Took all apart to leave today and no issues. Going down the road my wife went to restroom and no water pump. I had her try other switch and control panel over door is out. No inside power at all. Lights, pumps fridge all is out. Stopped and could not find anyting out of ordinary. Checked battery disconnect switch on panel over door and nothing. Still no lights. Cut on generator and in about 15 seconds power back on. Ran it awhile and cut it off and all went dead again. Went on home and hooked to shore power and it all worked again. Nothing was on including steps to get out door. If I ran generaror or hooked to shore it was fine but once I cut on engine only it all died. No issues running motor home or dash or generator, only cabin power. Help before I go broke as a shop please. I am not an electrician but somewhat handy so make your help simple please. Thanks for any help guys. Excited about first trip though it was great.