It isn't that difficult to disassemble to at least inspect it.
Back it up onto some 6"x6" ramps (for head clearance), drill out some rivets, and peel back the belly pan from the rear. (Wear eye protection and watch for fallout.) If the bolts are rusted and shear off (I assume you're referring to black tank supports), grind them off with an angle grinder, and drill them out if necessary.
If you don't own an angle grinder, you can purchase one at your local home improvement center for less than the cost of one hour of an RV tech labor rate. If you find that more is needed (i.e. welding), you can purchase a new or used welder (depending on size and capacity) for about 4 hours of an RV tech's labor rate. You get to keep both tools when you're done.
The tank is supported by the metal box
(two pieces of angle spanning the inside of the frame members) and is held close to the floor with the threaded toilet flange which screws into the inlet of the tank on top of it) The Thetford repair kit, which comes with a new stainless plate, is under $50 I think. (I've purchased two of them in the last two years and I honestly cannot remember how much they were.)
If the tank itself is leaking (which is doubtful unless it froze solid), it may be able to be repaired as well with a plastic weld for less than cost of new one.
If you get nervous and decide to pass off the repairs to someone else, you'll at least have an idea of what the problem is as well as made it easier for the repair person to inspect.