I would supect a valve is sticking. Guessing you have the HWH system. If so you can get in touch with them> They are good to guide you through diagnose. You can goggle dump valve problems but my computer would not open the HWH diagnose. I have a slow wifi where we are now. Hope this helps
My prior motorhome was a 00 Diplomat 38A. For sure you had to have the ignition switch in the ACC position. Now this is the part that is cloudy I believe the jack panel needed to be turned on/energized when dumping air.
I had a problem one time as you described and was going crazy. Think I quit trying to dump air for a bit then went back and energized the jack panel dumped air then leveled with the jacks. Not 100% sure like I said but think this was my problem.
Hope you find the problem and let us know the results.
BTW if its a fuse its located behind left front bay door (the one in front of the wheel).