Spray-on closed cell polyurethane is the very best you can do but is the most $$. Encapsulates everything - no voids. Seals the cracks/crevices you can't otherwise reach and thus provides a better overall R-value. Acts as a vapor barrier. Needs to be applied by a professional co. with all the right equip. & protective gear. Nasty stuff that sticks like sn*t almost everything - work area needs to be sealed off with sheet plastic to contain the tiny airborne droplets. Some insulation companies do it under a separate company in case they botch it up and get sued. Might get a better price if you can tow the shell to where they are doing a building job - most of their time is in prep & cleanup.
Closed cell foam sheets would be a good 2nd choice and can seal remaining gaps with spray can foam. Make gaps around sheets/pieces larger so the can's wand can reach into the gaps otherwise it may not adequately fill gaps. Could also use polyisocyanurate sheets (aka "iso" or "freezer" foam) which has bit better R-value - makes an awful dust when sawing it tho. A brief summary on rigid foam types
here. All 3 types have around the same R-value per inch, except full spray-on fills the voids. Use a dollop of PL400 on horiz. surfaces to keep pieces in place.
Spray foam expands a LOT and you'll be left with excess that needs to be trimmed with a saw or large knife (they have special ones for the pros). Off-gassing and airborne dust particles can be a significant health concern when working with it. Do some googling to protect yourself.
One reason wood framing rots is because the RV industry doesn't use a vapor barrier on the interior side of a wall/ceiling like in houses. Moisture in the inside air will migrate through batt & open cell insulation to exterior surfaces and condense in cold weather leading to rot/corrosion/delam. A 100% spray foam job will prevent this. For a sheet & spray can foam job *maybe* consider adding sheet plastic on the inside walls either in full or just where you know there are cracks for moisture to get through - but make sure not to end up creating a wall with a vapor barrier on inside and outside that will trap moisture within the wall cavity. Aluminum siding likely has a large enough space between pieces to allow the wall assembly to breath. Ceiling cavities probably should have no vapor barrier on interior side of joists due to impermeability of the roof membrane plus all the penetrations thru the ceiling's luan. 100% spray would def. be a good choice in the ceiling cavity. Whatever you do in the ceiling will be better than the factory cr*p jobs they do with batt insulation.
Use a thermal break on the outside surfaces of studs where possible. Foundation sill gasket would be a good choice. Comes in a roll and would need to be cut it into strips. About 1/8" or so thick and will compress somewhat. Keeping any switches/receptacles off and wiring out of outside walls would help (like they do in fiberglass sided RVs).
I'd also pay attention to the underside of the floor including sealing up penetrations through the floor. If you have the $$, go for a 100% spray foam job throughout. Downside to any foam method could be if ever adding wiring, piping, etc. in future.
