It's the only way to really find all the leaks. The vents, the TV antenna, even the drain pan on the Air Conditioner can contribute to leaks.
Here's a thread
I can't find the one where one owner DID make a do-it-yourself tester.
He closed and locked all of the doors, windows and vents, even sealing the stove fan vent with tape, then took the mattress off of his bed, cut a circular hole in the bed's plywood base that was about 1 inch smaller than a large commercial drying fan, set the fan flat on the hole, opened his outside garage door, and turned the fan on, blowing air INTO the Airstream. He went outside and started spraying soapy water. Found quite a few little leaks and one or two big ones.
There are companies which pressure test houses, and they might be able to do an RV too if you can't find someone near you.
MAKE SURE you use the right sealant on your leaks, and remember they can travel for several feet before they show up on the inside.
Good luck, Paula