I have a 17 gallon marine fuel tank I installed in the nose of the trailer. You can find them at bass pro or cabelas. I cut a filler hole and ran marine grade fuel hose to the tank. I also vented the tank and hooked up a fuel gauage so no guesswork. works great has for 15 years. The big problem I was having with the tank on the tongue was condensation in the fuel tank. water in the gas is BAD. people steeling gas
On my first trailer mount I had a guy that made custom auxiliary fuel tanks for RV's, trucks etc make me one. It was great, fit under the trailer right behind the tongue, with a fuel neck spout and locking gas cap on front of trailer. Guy disappeared and I can't find him anymore. Now I use the spun aluminum ones like $1K. Try an on line search. Off road People use them for sand rails, dune buggies, etc.