It is easier to check the frame from underneath. If you have to replace floor, and it's along the edge, which it nearly always is, you need to drop most of the stuff anyways to be able to get under the channel to run bolts up into the wall cavity.
See our blog here
on how two RV/AS rookies tackle a 1979 Sovereign resto!!
Just the photos
.. (sometimes you don't need the blog, just a picture worth a thousand words..)
1979 Sovereign International
2004 Ford Expedition Eddie Bauer
2 Golden Retrievers