The idea is to get a little bit of air pressure inside the RV and then spray potential leak spots on the outside with a soap solution looking for bubbles.
There is a product for dealers that uses a blower that attaches to a roof vent for this purpose. It doesn't take much - just enough air to get some flow out through any leaks.
What you need to do is to seal up the RV as well as you can, pump some air into it in some convenient way, and then start looking for air escaping where it shouldn't. A leaf blower might provide enough air or maybe you could rent one of those blowers they have for drying out wet carpets or something. You might need a bit of duct tape and some cardboard to fit a vent or hatch cover to blow air into the RV.
It might be easier to find an RV repair shop that has one of these things - see Finding Leaks
for some ideas there.