On your Jayfeather all flat walls are already full of insulation, except where channels might be cut for wiring and plumbing runs. The wall is made of a foam insulating board with a light wood skin glued to both sides, and a layer of composite sheeting on the outside. So anything you add to walls will have to be inside the trailer or outside the trailer.
I'm not sure of your roof construction, there may be possibilities there if it has been built up hollow. I know that my roof is a built like the walls, a slab of foam insulation with inner and outer skins.
One of your problems is that your TT is way too large for a single 13,500 BTU air conditioner, in sunny country. Another is that it is a lightweight, so it has been built with thinner walls than traditionally used for RVs built with foam core wall panels. Two inches is typical, lightweights might have walls an inch and a half to an inch and an eighth.
A lot more heat goes through the windows than through the walls, and most of the heat comes from solar radiation rather that air temperature differences. Thus reflective insulation is the best place to start, and the windows the first candidates for coverage. Another really good idea is to park in the shade where possible.
I learned how much the sun heats my RV (2 inch walls, 3-6 inch roof, 2 inch floor insulation) when I parked it closed up in the sun on a 70 degree day. It took only 3 hours for the inside temperature to exceed 99F, highest reading on my thermostat. Opening windows and using the vent fan to pull in outside air cooled it down to about 5 F above ambient.
