Originally Posted by akeagle
I have no advice about the lights, but you might try a decent electronic stud finder to find any crossmembers.
Failing that, a Harbor Freight metal detector should work or even a strong magnet since the crossmembers are steel. Also, Winnebago should be able to provide you with a detailed drawing if you call them. They're spaced more widely than you'd think they'd be. They sent me one when I was researching wall supports. In addition to cross-members, there are strategically placed metal plates in the ceiling and walls for mounting various things. These will also be shown on the drawings. I've also noticed that, on a dewy or frosty morning they can be visible on top of the roof.
You won't find the detailed info on cross-members, but there's a wealth of info for you here (parts catalog, plumbing and electrical diagrams, etc.):
Also, here (the 2017 is just the publication date):
I suggest downloading the plumbing and wiring diagrams from Winnebago and the HWH info so you have it when you need it even if you don't have internet access. And, if the info is downloaded to a laptop or tablet that you'll have with you when you travel, even better.