Tire sizes are denoted by not only construction and intended type, but ordinary physical descriptions: height in inches, section width, tread width, correct mounting rim size, etc.
In terms of changing from one tire description to another this is a handy way to make sure one has the correct size.
For what it is worth, I would not have 17" tires on my trailer (oem size 15"); I'd start again with stock sizing and make comparisons from there.
Travel trailers have very little suspension travel, overall. Even an Airstream.
Tire Size Conversion Chart
Tire Tech Information - Diameter Comparison of Light Truck Tire Sizes
Tire Tech Information - Load Range/Ply Rating Identification
I was earlier reading an AVION manual and noted that the tires recommended in 1972 on 25' and 28' trailers (6800-lb GVWR) were H78-15. This was a tire that was, according to COKER TIRE, about 28.3" in height, the same as a 225/75-15. I cannot find a "true" old chart to give me the exact info on tire height, but it should be within an inch.
The second note was that AVION wanted the tire on a Load Range C tire; the obvious replacement would be a GYM in 225/75-15, L-R C as the weight rating is consistent. The same size L-R D would not be. The first is a 50-psi tire, the second a 65-psi tire.
Of course, a trailer tire should be mounted on a rim with a weight rating well above what a conventional auto wheel would carry.
On my '83 Silver Streak I cut a finer margin with a tire that was a full one inch
taller than an H78 tire, a 7.00x15 Yokohama that I felt was better built than the GYM.
There is bound to be a truck tire service center in your area where the farmers or service contractors bring their small trailers in for tires, and could be of help to you (maybe some old rims you could use).