I've been focused on insulating the trailer and the end is in sight.
As I'm planning on mostly boondocking in the mountains and deserts of the southwest, I've got to make this thing habitable in extremes of heat and cold with no option for plugging in to grid electricity. That means no A/C and no traditional RV gas heater.
So I need to insulate it well. In a perfect world, my body heat will be enough to keep it survivable through a cold night. What do I mean by survivable? If I can keep the interior air temperature above freezing on a 0*F night, I'll be happy.
My strategy for summer is to have fans, excellent roof insulation, a good outside shade hang-out area, and the ability to simply weigh anchor and move to higher elevations if it gets too unbearable. I'll consider small, portable swamp coolers, but most of the ones I've seen seem pretty poorly made. (It's important to note that I'm planning on being in warm humid climates approximately never. The design decisions I'm making are very much driven by an arid-only assumption.)
The first and most important thing to understand about insulating structures is that heat likes to cheat. If you stuff the cavities with insulation but your outside surface is aluminum and you have metal studs, the heat is basically going to ignore your cavity insulation and go straight through your studs. This is called "thermal bridging".
In September, in Santa Fe, I had just put up wall cavity insulation. The metal stud was literally too hot to touch. In order for this trailer to be remotely habitable in warm and sunny weather, it needs to somehow insulate these thermal bridges. So in the walls and ceiling, I will have two layers of insulation: cavity insulation, and a mostly continuous layer of insulation that covers the studs and ribs.
For the floor insulation, it's a little trickier. I don't want to add continuous insulation below the floor joists, because I want to maintain my ground clearance. And I don't want to put continuous insulation above the subfloor, because then I won't be able to stand up straight inside. If I had a taller trailer I would definitely do indoor continuous insulation... but if I had a taller trailer, I'd get worse gas mileage and have more wall area to transfer heat, so my thermal performance might even be worse. It's all a trade-off.
I decided to do 2" cavity insulation and then use pipe insulation to cover the joists. This will help mitigate thermal bridging in the floor.
I used polyiso insulation because that's what my local hardware store has. In retrospect, the foil is very thin and clearly won't hold up to thrown rocks as well as XPS. Once the foil is damaged, the exposed polyiso will be vulnerable to moisture and just powderizing and sort of falling apart.
If I had it to do over again, I'd use XPS because it just seems tougher for this use. That said, I don't expect this to be a significant issue. I'm not concerned about moisture damage because I'm almost always going to be in arid regions -- even if it does get wet during a storm, it'll dry out almost immediately. And as for the rocks tearing the insulation up, I'll just keep a roll of foil or duct tape handy and inspect it often. If it gets real bad, I'll protect the insulation with some other membrane, but I suspect I won't need to.
To secure the insulation, I dropped bolts from the subfloor above and snugged it with a metal strap.
A possible bonus of underbelly insulation is that it might improve the areodynamics of the trailer, and thus improve my gas mileage. Every little bit helps.