Checking for leaks is wise! You can have the system pressure tested for leaks in the same manor a plumber will check a house system (except at lower pressures in a gas system). Most of the time, it is just the “odor” that permeates through the tank.
All plastic tanks have odor permeation. It is a nuisance odor that is present all of the time especially when the vessel is enclosed (camper back, etc.). You can look on the Moeller Tank web page and find the data sheet the states that all plastic tanks have permeation of odor.
“Odor” vs. “Fumes” is sometimes dependent on the sensitivity of each person’s ability to smell. The majority of the time it will be just nuisance odors! But check it out to confirm.