Just to set the record straight. The newer pop up truck campers do not use canvas and I have never had a problem lowering mine with a wet side wall.
As to the construction, different makers use different designs. For example, my Outfitter has a basement and the tanks are there. Northstar pop ups do not have a basement and many of their units have cassette toilets which eliminates the black tank.
Your best bet is to look at as many units as you can, because only you can decide which fits you best. One great resource is Truck Camper Magazine's buyers guide.
Buyers Guide.I get 13 - 15 mpg with mine.
Good Luck