PSI

I respectfully disagree that running max PSI is bad advice for an RV. Dramatically running over pressuring the tire isn't good, but you're making a compromise when setting tire pressures: ride quality will be better with a lower pressure and be harsher as you increase PSI. Adding more pressure theoretically reduces your contact patch, but unless it's drastic, you won't notice a difference. Perhaps higher pressure will wear the middle of the tires more.

But given the higher failure rates for RV tires vs regular tires, I'd rather run max PSI and have the extra load capacity, at the expense of perhaps a slightly harsher ride and wearing out the tires a little earlier. Even then, I've had to replace RV/trailer tires due to age before they wear the treat out.

In the end, I don't think it's wrong to run at the exact tire PSI for your RV weight, but I also think there is merit to running at max PSI, which is what I do. And I don't think anyone will argue that it's good to run underinflated (unless perhaps if you're on sand, before some smart ass tries to make that argument).

Max PSI is bad advice. The correct way to do it is to weigh the vehicle and use the tire manufacturers weight/pressure charts to determine the correct pressure.

From the Michelin RV Brochure:

THE IMPORTANCE OF TIRE PRESSURE
The most important factor in maintaining the life of
MICHELIN® RV tires is making sure they are always
properly inflated. Incorrect air pressure for the weight of
the vehicle is dangerous and could cause things like
premature wear, tire damage, or a harsher ride.

An underinflated or overloaded tire will build up more
heat that could go beyond the endurance limits of the
rubber and radial cords. This could cause sudden tire
failure. Underinflation will also cause poor handling, faster
and/or irregular tire wear, and can decrease fuel economy.

Overinflation, on the other hand, will reduce the tire’s
contact area with the road, which reduces traction, braking
ability, and handling. A tire that’s overinflated for the
weight it’s carrying is more prone to a harsh ride, uneven
tire wear, and impact damage.

Tires. Better to have more than not enough as long you stay within the max psi indicated on the tire. Don't worry about the tire wear as a result of running them at max; you will probably need to replace the trailer tires way before tire wear will become a factor.

I always use the max psi that the tire are rated for, so in your situation I would use 65 psi. A tire's load carrying capacity is tied directly to pressure, and I prefer to take full advantage of that by pumping my tires to the max.

As for the post above about weighing your TT and setting tire pressures accordingly, again I go to max PSI for the extra safety margin. If you lose a tire, your remaining tires will have to carry extra weight so I don't want to be at 'just enough' tire load capacity.

My take is that the trailer came with a load range C tire (50psi max) and the tires you have are load range D (65#psi max) given this I would say that I would go with the tire store's psi, if I didn't go with the 65psi on the tire. JMHO

None really. You need to go to a scale and get loaded weight then go to tire manufacturers charts for correct PSI.

Tires