Your tire isues is typical of the usual ST tire story as many folks have found out the hard way. The Towmasters are used all across the trailering world (not just RVs) and every website shows the same type problems with many owners.
With 6860 lb gross axle means a minimum 1715 lb tire requirement.
The Goodyear Wrangler HT LT215/75-15 D load range at 2095 lbs and is 27.8" tall would make a excellemt replacement tire if you have 15" wheels.
Lots of USA made LT235/75-15 C at 1985 lb but are 28.9" tall.
ST tires up to 16" come in ..
load range C at 50 psi
load range D at 65 psi
load range E at 80 psi
load range F at 95 psi
