I would suggest you try to blow the air into the cabin rather than pulling it from the inside. You could probably reverse the vent mount and attach the blower to the hose, outside, and get sufficient pressure, blowing in to pressurize the interior.
I used a squirrel cage furnace blower mounted to a window. I drilled a hole in the plywood mount to accommodate a 1/2" plastic tube. The tube is part of a DIY manometer that will show how many inches of pressure are being developed. The Sealtech system develops about .5 in. of differential. My squirrel cage blower measured 1" differential or more.
A pin hole leak discovered using dish soap and water:
Good luck with your leak fixing, it sounds like you might have found the biggest one.
