We find our 13.5 BTU A/C works fine for our 30 ft rv in 100 degee temps. Here is what we do:
1. Park with the short side with the smallest window into the afternoon sun. For us it is the rear bedroom window. If sun is going to hit the side as it goes through the day position the RV so the awning can help protect that side.
2. Put the insulating foil in all the windows (including the front windows). You can get it at a hardware store. Here is what it looks like.
Multi-Purpose Foil - Product - Camping World
3. Put the insulating foil in your roof vents, shower dome, and over your roof fans. We also have a piece that goes over the vent out from the stove top. We use the sticky backed velcro which allows us to put the foil in place and take it down when not needing it.
4. Remove any food from the cabinets along the roof that would be hurt by the heat and then keep the cabinets closed. They provide an additional layer of insulation.
Good luck! I don't envy you being in the RV in this heat.