Unless you have a hard requirement for A/T tread for lots of mud, for example, then street tread is not only quieter on pavement but wears better. I've been on the road 6.5 years and 130k miles. Once my stock LRC P-rated tires wore out I went to LRE LT tires. The trick in my case was bumping from 265/70R16 to 75 aspect ratio for market availability of LT choices and LRE rating. I went Michelin LTX2 (no regrets) but other choices abound. My truck's always loaded so I can run these 60 instead of 45 psi. It feels the little bumps more, yes, but the truck handles better and rolls along easier with a bit better MPG. And they're wearing favorably - slowly and evenly. The 75 tires were a couple percent larger OD than 70 which also corrected the optimistic speedo cal. so that was a bonus. As for street tread I've only been in soft sand, dry washes, rocky canyons, slushy snow and dicey ice a minority of times but I've never had a problem with traction. I also hope 10 ply rubber plays Murphy insurance against roadside assistance; and so far, so good, knock wood.
As for box design I did my share along similar thoughts before and during this chapter of my life. I even had a four-wheel pop-up on my Tacoma before it was suceeded by a Tundra. But I set off full timing with a trailer camper. And while I've stuck with the Scamp brand I downsized from the 19' fifth wheel to 13' std hitch, mostly for bed cargo reasons. I still noodle over slide-in designs but at this point (early fifties) I'm less enthused about a scratch build. In my future I can see a Bigfoot slide-in but on a heavier duty truck with larger/dual fuel tank for extended range. I'm less obsessed with record MPG and more focused on the happiness meter. YMMV
Any RV is eco friendly compared to houses and even apartments or condos. And micro campers leave the smallest footprints. So my advise is to think through YOUR use cases to develop the requirements that best meet your needs and wants. Shaving box width or height, tapering rear, and rounding front edges are excellent ways for less less wake and drag on the road. Just make sure that once stopped to enjoy the world, that you're not too stooped or cramped, for example. Make sure you budget enough weight and space capacity for food, clothes and gear you really want to enjoy. The more of those corners you can see around now, the better things can go later. I also hope you thoroughly enjoy the build. Right now that's probably the fun part.
...and keep us posted - before, during and after!
All the best
Bob