Yes, ST205/75D15 tires would be able to replace your ST205/75R15 tires as they are the exact same size, with the only difference being the D tires are a bias ply, and the R tires are a radial.
I recommend sticking with a radial tire like the Karrier ST205/75R15 Radial Trailer Tire - Load Range D # AM10245 as they are going to give you better on-road traction due to the design of the sideway, and will stay cooler on the highway leading to longer tire life.
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I definitely recommend going with a new AC unit, and for a travel trailer a 13,500 Btu unit will be plenty. You will also need a start capacitor in order to help jumpstart the AC. The start capacitor will actually store power so it doesn't draw as much when you are first starting the air conditioner.
For this, I recommend going with an Advent AC unit:
- Advent Air RV Air Conditioner w/ Air Distribution Box and Start Capacitor - 13,500 Btu # ACM135C
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