One of the ways I find some "random", "small", "a drip ever once in a while" leaks of all kinds is the paper towel method.
lay some clean white paper towels flat under the tank especially around seams.
wrap / tape them around the sender and joints. make sure its flat on the seam/seal of the sender.
Same with filler neck.
Let them sit for a while and then check them in a few days. carefully slide out the ones under so you can line up the drip with the tank if its there.
If there is a drip it should stain the towel even if it evaporates.
Cardboard under the tank will work for this too.
This works best with a full tank to the top. so its pushing out liquid.
since you thought it may be a pressure issue originally with the change of paper/rubber seal of the cap.
I would start with the cap that may be holding pressure or not venting. Put truck in sun in let it build up pressure this will help make the leak happen. "SLOWLY" open the cap after some time.
You could clamp the hose going to pump under the truck. you can also manually put a SMALL amount of pressure in the tank with a air pump. I used a power steering / radiator vacuum rubber seal kit and pumped in.
Since the smell is inside the cab it should not be a cap issue unless the cap is not releasing pressure which this test will tell you.
