The aluminum rims might be marked at 60psi maximum, but that is for load factor, not the additional 5 psi you're considering. You're not increasing the weight of your trailer, just the air pressure.
Goodyear recommends running ST tires at their maximum rating, though on a lighter pop up, it could create premature tire wear.
Going with the steel valve stems is a great idea. They don't rot like the stock rubber ones and they hold air better.
Also, consider balancing your new tires as it will help the ride quality of your trailer.
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2007 Dodge RAM 2500 6.7 Cummins
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