Sir, I think a lot of your calculations are interesting and thought provoking. ... I think that, as one person put it, there are way too many variables to even remotely come close to guessing how much propane any particular family and camper might use.
I do think the new Viking composite propane tanks that do offer campers the abilty to visually check the level fixes the age old question of how much propane do I have available..... To some a relatively small cost for such a feature.
2005 Dodge Ram Laramie, SRW
5.9L Cummins Turbo Diesel 24 valve
2013 Forest River Thunderbolt XLR 380 AMP
2010 Star Motorcycle Stratoliner Deluxe