Since you're going to be in the same location for a spell, I'd suggest to install a window unit in the trailer. That can be as easy or as complicated as you'd like to make it.
If it was me, I'd purchase the largest (8K BTU?) that fits a window in your trailer. I'd also install a separate branch circuit/circuit breaker/shore cord so that the window unit operates separately than the main A/C and other 120V loads. You might get lucky and find that a new, highly efficient 5K or 6K BTU unit may run while plugged into an existing receptacle, operating along with the existing roof unit. Some of the newer window units are very efficient.
About insulation: There is little that you can do without looking janky using exterior treatments. I know that I would deploy shade if at all possible. FWIW, I tore my trailer down to the studs and spent two weeks-full time installing new insulation. It is one of the best projects I've done. Besides the total silence, it is easy to heat and cool. BTW, painting the roof with new ultra-bright white roof paint lowered my interior temperature 10 f.
