savings in customizing a Ram ProMaster vs buying a Travato

A good idea for what the OP wants, why waste money on things you dont need.

Resale time, just strip the van back to basics and sell the van on the contractor market.

Much easier to sell than a Class B RV starting to show its age...

The Promaster seems a bit low on ground clearance in the stock form, add all the tanks, pipes, valves, generators etc from the RV builder and it might get a bit scary underneath.

A simple build would keep everything above the floor..

Also consider the Ford Transit van, more truck like...