Need a Stud Finder

Old carpenter trick... with your middle finger (biggest and heaviest), tap on the surface continuously as you move across. Hollow sound is obviously not a stud but, as the pitch goes higher, you're approaching a stud. Tap back and forth over the high pitch area and you have found the center of the stud. Easy to map out your supports with this method.

I have a Zircon I bought on Amazon. It had good reviews.
It doesn't always find studs through 1/2 inch drywall.

See if you can borrow one before you buy, especially if you're only going to use it to find the studs on your RV.

If you buy, get a good one and try it as soon as you can. You may have to return it.

And, if you get one that does the job, let us know the make and model.

I bought one at Costco to replace my beat up cheapo I have used for 20 years.
Would offer the old one but it is not worth the shipping.

I'd think most any stud finder would do that. It's not much different than finding wooden studs under gypsum board. (One exception: a few really cheap so-called stud finders are just very thinly disguised magnets that are finding sheetrock nails or screws. They won't work very well to find wood studs glued to fiberglass for obvious reasons.)

Can you go out in the morning when the dew is still on it? That will show where all the framing is from the outside.
Otherwise stud finders are cheap, give one a try.