Guys if you have look on the starcraft 17:58 thread I posted up some picks of where I found uprights.
http://www.expandasdownunder.com/th...-3-options-quirks-post-them-here.5166/page-21
Basically on a jayco you have a 25mm alloy frame goes completely around the circumference with two uprights each end , the front one drops where the roof line rolls in if that make sense?.
A quality stud finder might work in the fiberglass vans, or from inside? The other thing is if the speakers are going to back onto a cupboard or under a bunk. Drill a few small holes from inside to gauge what's there, you soon tell between solid wood, alloy or foam. And there not noticeable under the bunk if you don't use them.
Maybe like this but not as much. Once I knew nothing impeded it I drilled a small hole right through, checked outside and then if using a holesaw drill from outside in till I was half way then drilled from inside out. It minimizes chipping the timber .