Do like the pros do and put a good sized fan in an open window blowing outside. It creates a negative pressure inside. Air will come in any leaks you have. To find them you may be able to feel them, but if not, find an incense stick and light it. Move it around very slowly watching the smoke. You will see when you are close to a leak, the smoke will tell you.
Naturally you must have all windows, doors, vents and heaters turned off.
Happy Glamping.Norman, 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back. Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!