Sorry Jay, my bad, a TC is a Truck Camper and a TH is Toy Hauler. A TC being the most compact style with full amenities RV (which you can unload and then go launch your Island) or Toy Hauler is a big trailer with a raised roof and garage door like how you wanna use a box truck. A TH is what I would roll with if I had the bucks and frequented places for RVs. A TH is what we would get if we didn't have our large dog (small footprint and no large trailer drama).
Good to hear you have that sailing experience and liveaboard too. You are no doubt well qualified for making the plans you are making.
Regarding the box truck living and Island storage......I guess, no offense here, but maybe apart of me suggests you tow, besides the obvious, because it may attract more company.
Being sandwiched between my guy and a kayak in a loft bed doesn't exactly sound appealing. Although maybe I am the last kinda gal you would want next to you anyhow, ha!. We are rather experienced and well versed in the liveaboard/sailing community and I know some guys wishing they had ladies in their lifestyle and, my observation, is some most likely don't, despite being great men because their boat is too small or not enough amenties or are too dirty a lifestyle (bigger boat and more amenities usually fix this issue). Just keeping it real here.
We actually considered converting a box truck ourselves, so trust me I have a glimpse of what you are looking at. This is just my opinion and we all know the saying about opinions and a$$holes....everyone's got one.
So here's more of mine...
Schedule and freedom galore does not make up for a busted back from wrangling it up on top of an suv let alone higher in a box truck. The only way I see it as being doable is by electric winch. I remember a dock neighbor taking off to go cruising, fit guy about 50. It gave us the chills seeing him take off with a 15hp o/b on the rail with no o/b hoist or anything to help singlehand it off. He was pretty macho and cavalier about it but a neighbor with many thousands of cruising miles along with mine gave him a speech about getting an o/b hoist stat, otherwise his dream, his 3 year cruise could end very quickly, very fast. Suggesting the trailer is a bit like that, unless you get an electric winch (which you could) And/or a block and tackle with a lot of purchase, plus big wheels or a dolly.
We actually bought our TI with the intent of car topping it on an RV with an electric winch. We ditched this idea once we owned a TI (and together we have well over 30 years sailboat experience so we really talked it over).
You're an engineer, you know how much heavier it will be elevated and trying to move it over your head and getting it there in its spot. That's why everyone keeps saying trailer, it's a total pain, like a major pain, even for us with two people. I guess worse case you try it, if it doesn't work get a trailer so no big deal.
Good luck!