Since I repaired my Thetford toilet myself, I know what a chore it is.
However, it is very easy and you can do it yourself.
In fact since you have it apart, you can actually put some upgrades in at the same time to make your "Hopper" the coolest one in the park (Unless I am there too).
I am worried you might have damaged your valve by trying to blow air through the water inlet. There is a tiny hole in the flush line that can be blocked by a granule of Calcium or bit of anode (sucked into the cold pipe from the water heater due to pressure changes in the pipes when the cold tap is opened).
See what is involved in this thread:
Repairing a Thetford Aqua Magic V
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW