retired1, here are few more thoughts. It would appear your camper likely weighs in the neighborhood of maybe 6000 to 6300 pounds or so. If you have actual scale weights, that would be good info to have.
I just looked at tha general Keystone RV owners manual, and on page 22 they state that the trailer may be factory equipped with either a ST tire or an LT tire. To me, that says they approve the use of LT tires on their products.
Looking at the photos of trailers like yours, it appears the wheels "may" be Dexstar brand wheels. This wheel brand idea is just a guess on my part. You could verify that likely by looking at the wheel, or calling Keystone. If..... key word there....."if" they are dextar, check to see if they are a model number 017-142. Looking in the Dexstar catalog, that is the only part number they list in a 14" size for the style I saw in the Keystone ads. Dexstar lists that wheel as being a 65 psi wheel. If so you are good to use load range D tires.
Here is a "for instance" for you: a Kumho 857 LT tire in a 205-14, is rated at 2271@65 psi. That is 9084 pounds of load capacity at 65 psi. If I am right that your trailer weighs aprrox 6300 pounds, and assuming 12% of that is tongue weight ( about 750 pounds ) then you have approx 5550 pounds on the tires. That gives you a very comfortable tire load capacity reserve of 39%.
There are other options in tires. I only chose the Kumho because it is an example of one tire that is available in a 14" size. If you have room in the fenderwells for larger diameter tires, then you could consider moving up to a 15". Obviously this adds considerably to cost though having to buy new wheels. If it were mine, and I could comfortably raise the load capacity numbers with the existing wheels, I would do so.
Hope this helps, and hopefully you will solve the issues. No fun at all having to deal with side of the road breakdowns.