Locating roof crossbeams for solar panel install?

I tend to agree with Fiverwheel, except that I think there would be less flexing if you aim for the roof trusses. Flexing is not necessarily bad but could hasten a leak scenario. I bet your 2003 Prowler is built similar to my 2004 Pioneer, which means they're every 16" on center. As previously mentioned by dockmasterdave, you can easily feel where the wood cross members are. I also think I successfully used a stud finder.