As stated a number of times above already, we have carried 5 gallon gas cans for years in the bed of our truck under our metal rolling tonneau cover. We did on two occasions need to add to our truck's fuel tank to ensure getting to the next gas station, other than that we use it to fill our Champion inverter genny.
Also carry firewood, charcoal and other items in the truck bed that would certainly make for a heck of a fire should gas contaminate them, alas never an issue. As long as your rack on the rear of your TT can handle the weight and fuel can be securely mounted, as it sounds you have already made your decision, give it a shot and see if it works out.