I changed from 205 75 R 14" Good Year Marathons to 15" Maxxis tires and 15inch wheels after several blowouts on my '97 3023 TM. The following website was most helpful in measuring the rims to get an exact similar rim one size larger. The site is:
https://www.rsracing.com/tech-wheel.html
This site on wheel technical information discusses how to measure bolt patterns and how to measure back space. The backspace (distance from highest part of bolt circle to top of outer lip of wheel) on my 14" rims was 3.5inches. This means my offset was "0" as indicated in the Backspace to Offset Conversion Chart. I purchased two 15" wheels by searching for them on the Internet. Look for Trailer wheels.
The M 8008 ST Radial 205 75 R 15 Maxxis tires (part No. TL15703000) are 1.1" taller than the 14" Good Year Marathons. They are 8 rather than 6 ply and rated to carry a load of 2040 lbs at 60 psi and a load of 2150 at 65 psi. At 65 psi is load range D. I sure hope these tires are a better fit for our well engineered and cared for TM. Load range for the Good Year Marathons was 1760 lbs per tire.
Since I have had at least one, perhaps two blow outs due to poor valve stems made in China, I opted for all steel valve stems. They are solid and available. (Nappa Part #90-416 Tubeless Clamp-In Tire Valve TR 416
1 and 1/4" lenght, Fits .453 and .625 rim holes).
Hope this info is helpful to anyone looking for a tire-wheel upgrade.
Happy camping!