Trailer Tires, Need Help

I have a 2000 pro air on a 1999 Prestige trailer. Single axle. I would ask them, but apparently they've been out of business for some time. Not a great sign.

What load capacity tire should I have on there? They're 225 75R15, and the one's I have (which I replaced all 3 of them this summer already) are rated at 2500#, but I figure I'm probably close to 4500# with the boat on the trailer. Any time I hit a little bump in the road the sides of the tire bow out like crazy, and any time I hit something substantial I have visions of changing them on the side of the highway again.

I talked to the guy at the tire store and he was about worthless, told me I needed to know what kind of tire the trailer was rated for, cause if its C I can't go up to D, and not D to E...whatever. This means nothing to me. The manual was no help. The only intelligible markings on the trailer are these, and I get nothing from them.







I figure I got about 36 hours of road time I'm going to be doing with this trailer in the next month, and the last thing I want is to be on my way to Tim's, changing a tire, listening to "I told you you were going too fast"

Somebody with a similar trailer or boat help me out. And yes, I know, the obvious answer is to upgrade to a tandem axle. That's not happening before next weekend.