Sounds like a cool project Kayteg and from reading your posts on here for a while, you have the skill to do it.
I’d def go with the sprinter vs the box van, just for asthetics if nothing else. They make dually sprinters too but if fuel economy is a top goal then weight and 2more tires on the road are not your friends. Better aero out of the sprinter too. But you can’t get blood out of a turnip so I wouldn’t be banking on 25mpg. Plus your comparison of $1200 for 2000mi in your dually is off by a factor of 2 at least. You’re more like $500 in fuel for that distance.
Regarding the van, IMO, if no running water and shower, fridge, decent power setup, it’s not worth it to make a bed on wheels with a porta potty. At that point , you can get another TC for cheap that isn’t rotted out and needs nothing. My last 2 AFs were $12k and $10k respectively and neither depreciated, yet. Money is better spent there,but no satisfaction inbuilding your own rig. Headroom and figuring out the tanks and shower imo are the 2 biggest challenges.
Don’t try to do a slide out. Imo that’s a recipe for failure with a big @ss hole in the side of a van. With the sprinter. Box van, Is attempt a slide out, would be bad @ss!
Good luck, let er rip! I’ll send you some sawzall blades! Lol