Thor Avanti renovation is very doable. Jackknife sofa can replace dinette, vinyl plank flooring is popular, and mattress can be replaced with standard size in most cases.
Your comprehensive rebuild plan is solid, but focus on structural integrity preservation, proper moisture barriers, and weight distribution. Critical areas include electrical safety, plumbing rerouting, and maintaining slide-out mechanisms.
Your 1991 E350's symptoms don't clearly indicate oil pump failure - the consistently low oil pressure combined with low operating temps suggests possible gauge issues or sending unit problems rather than pump failure.