Regarding your fuel consumption questions, I can only give you our experience:
2002 Winnebago Journey 36' with 330 hp CAT, we averaged 7.5 mpg on most of the roads we routinely traveled. Less in mountains but 7.5 overall.
We now have a 2007 Country Coach 42' with tag and a Cummins ISL 425. This coach weighs nearly 6 tons more than the Journey. We get 8.2 mpg traveling the same roads.
We love the 42' coach and have no regrets. We have thought about where we enjoyed staying during the 8 years we had the 36' coach and can not think of any where we would not fit in this 42'. There might be a few but we could not think of them.
As noted, the usual limitation is the narrow roads and/or folks who park their stuff in your way.
'07 Country Coach Allure 470 Siskiyou Summit #31578, Cummins ISL 425; 2014 Ford F150 toad; Air Force One Toad Brake.
Glen Allen, VA; Smith Mountain Lake, VA.