You will also want a whole house filter, sedement and charcoal. They are cheap and readily available. Reverse Osmosis is optional and probably not in your budget. Full timers often have water softeners as well, especially in some parts of the country where the water is so hard it damages the plumbing.
Let us know how you come along on keeping the conversion to under $8K. Sincerely, we need more data points on what can be done on the cheap.
You might also check out this site:
Cheaprvliving.com
I found it before I found this site, and it was the inspiration for the Stealth Camper. The section on living in a box van was especially helpful. Their mission is different, fulltiming in a small space in mostly urban environments, but they share your desire for low cost and the space is similar after you subtract out for your shop area.
Actually, what I think is most similar is some of the toy hauler setups that the racing guys use. You really should check out racingjunk.com before you spenda lot of money. (I know, you already own the truck.