A long time ago we owned a 1972 Dodge B300 Family Wagon Camper Van conversion. It was easily the most versatile vehicle we have ever owned. We were young family with three kids and had moved up a little from a VW camper conversion. The van camper had a two burner propane stove, an ice box, and a sink with water pump and a water tank, It had a small potty room with a porta-potti. The bed/dinette was convertible by moving cusions etc. There were 12 volt interior lights and no air conditioning or furnace. There was an overhead bed for kids and another telescoping bed in the rear. It had a bubble top which allowed standing up inside. There was very little storage room except for the front overhead bed. You could easily cobble a rig together like that. It got about 16 mpg and was usable as a hauling vehicle and daily get around. Today's Roadtrek van conversions have all of the comforts of home packed into a van. They are custom produced in small numbers and are probably not as quick/easy to assemble as Class C's.
Best to go to a Roadtrek dealer and simulate using the bathroom/shower, dinette and bed, sitting accomodations. You can plan on sitting outside under the awning while camped for "elbow room".
MPG won't be so great because of the all-up weight. Maybe a used Roadtrek or similar van conversion.