Probably the most significant reduction in heat gain you could achieve at least cost is shielding the trailer from direct sun. Just stretching a tarp on a frame of pvc pipe blocking most of the sunshine will be inexpensive. Insulation will be pretty cost ineffective in comparison, however I don't think you shouldn't insulate. I made a trailer without insulation in the PNW years ago and spent my first night in it in winter. That morning I awoke it was 32 F. That day I installed 2" rigid foam inside, and the next morning I awoke to 72 F, while the weather hadn't changed.

Particularly since you have a trailer which is intended to be mobile, insulation will prove a good investment IMHO. Also, while when stationary that frame appears robust enough to nominally support the weight of the AC unit, it will be a different story when the trailer is bouncing over potholes.

Good to see folks DIYing stuff I wanna make too =)

Thanks!