Wonderful--Toilet Issues

After having several 5th wheels in our camping life, we decided to try out a motorhome. Our reasons were primarily they are easier to back into a site and maybe drive, I'm questioning this decision. We have had our used 2013 mh for two months and have had no fewer than a dozen issues. Every time we stop at a campground I'm working on something. We full time in it so once we get to a cg for an extended time it has to go into the shop. This morning the toilet overflowed, Good Morning!!, it appears the black tank filled itself to the point water came out of the toilet and of course all over the bathroom floor, hallway and into the kitchen. The black tank vent filled with water and water was pouring off the roof. Its suppose to rain today which hopefully will wash the roof off. We are starting to think the camper is haunted, we've never had this many issues with any of our 5th wheels. Back to the toilet, now the toilet bowel will not hold water, whats going on with this? Do I need to replace the toilet or is the ball valve(I don't know what else to call it) in the toilet need to be replaced? How difficult is it to fix what ever is causing the water to leak out of the toilet? We have cleaned up the camper with fans blowing to try and dry things up. We have used the toilet, sink, kitchen sink and cloths washer and everything seems to be operating fine. We have been on this site for only 2 days so I know neither the gray or black tanks were full, I don't know what happen, everything seems to be working fine and no leaks. The last thing I did with water was last night I filled the fresh water tank to 1/3 level. The cg water pressure is low so we have the onboard water pump on along with the cg water on. Off to fix another mh problem! Thanks for any feedback on the toilet ball valve.