So it sounds obvious now you have a problem.
Sometimes (actually, it seems more often than
sometimes), it's easier and better to do it yourself than to hire someone else to do it, especially when you have already exerted so much effort into trying to get someone else to fix it, and what you're dealing with isn't a life or death device. I have zero formal vocational training, and for some reason, I find myself frequently giving advice to contractors, particularly auto mechanics and plumbers. Not on everything, of course, but little bits here and there....like making there there are no tools next to the fan belt before closing the hood and sending me off....and suggesting that perhaps it's a good idea to drain the old 50-gallon water heater before trying to drag it out of the basement. The only reason I do it is so that I don't have to do over what I paid them to do, or deal with some other negative consequence.
The toilet may look big and intimidating, but it really is a very simple device. It's just a glorified bucket.
I understand you may not be the handiness folks around, but neither were any of us until we started trying to fix stuff.
If I were in your shoes, I'd print out the toilet diagram at the Thetford site above for easy reference. There are literally just 4 phillips screws required to access the guts of the toilet, which have been described here. It would literally take you less than 5 minutes to pull it apart. Once inside, explore around a bit so you can figure out how the thing works and also where all the water flows (so you can tell us
). I expect that whatever the problem is will be obvious once you look inside and learn where the water flow is.
And a very nice thing about learning how stuff on your TM works (and your car too) is that when you are out in the middle of nowhere on a Sunday night, and something breaks, you are not nearly as dependent on someone else to get you out of a jam.
So if you do decide to venture under the lid, just pay attention to what you take apart, take your time, don't force anything, and if you need help, come back here.
Dave