That SimpleTire listed above has TransForce in 8.75 and I'm not sure but they dropped the 8.00 size when they went radial. IIRC the 8.75r16.5 is about the same size and rating as the 225R16 used on newer Class C's.
TireRack.com has TF's (called TransForce HT) in 8.75 but not 8.00 so I'm thinking the 8.00 is really getting rare. Firestone's own website was useless. A "Dealer" (???) who doesn't know about a tire of their manufacture and can't look it up probably doesn't want to deal with the hassle of a motorhome when you probably won't be buying a brake job, alignment, etc.
See if there's a Discount Tire in your 'hood. Most of us have found them very helpful.
You DO want a radial. We replaced 8.00/16.5 bias tires with 8.75R16.5 on a van years ago. Van wasn't dually so no spacing issue. With bias tires, it felt like the wheels were square the first few miles. I couldn't push it out of the garage with my butt against the bumper shoving with my legs. With radials, I could stand behind it and it'd roll when I pushed with one hand. It weighed 4500 so it wasn't an easy push, but you can see what I mean.