Possibilities
1) improperly sealing entry/ cab door/s. Take a dollar bill and shut the doors on it. Try to pull the bill out. It should be very snug. Now try this in a few more areas of the door. If the dollar pulls out with little resistance, the door is not well sealed.
2) do the same thing on each window. Very often, there is a seal where sliding windows meet when closed. Most often this seal is not air tight. No easy fix for this very common problem.
3) Crawl underneath. Look for any place where a wire or pipe enters the coach. Is the area well sealed? If not, a can of aerosol spray foam might be the answer.
Some RV places have a way to seal an RV them pressurize the interior. Then they spray a soapy solution around and look for bubbles that indicate escaping air.
Or, do as we mostly do. Go boondocking where your nearest neighbor is a mile away.
Enjoy your travels
Ed
