Go ahead and replace the 14" wheels with 15" and run load range D (8 ply) tires. I think the Power Max are Chinese tires like most off brand tires today. I had an Airstream a few years ago and had problems with the Goodyear Marathon tires and went with an exact tread look alike Chinese tire and solved the tire issues. My current Casita
17' has the GY Marathon 15" and I will replace them as soon as I have any type of problem with them. I think all trailer rated tires are rated at 65 mph max.
I am presently running load range E (10 ply) on my Tahoe, made by Cooper and called Master Craft, and have great luck with these and would buy them again.