yea. I believe it's pretty common for SUVs/pick-ups to be custom-fitted with extra fuel tanks especially in southern or mid-western area because the "nearest" gas station is usually about 30 min away.
every 2 years, we have IRON BUTT RALLY competition where motorcyclists have to complete 18,000 miles in 18 days all over America. their motorcycles are custom-fitted with extra gas tanks (just like pix above) in the rears.
(silver rectangular metal box under the black compartment)
(the black container with slanted corner)
when I was at my mechanic's garage last year, he had a client's bike (also iron butt racer) custom-fitted with a beer keg for fuel. all perfectly legal - assuming that everything meets DOT and EPA regulations.
Fuel Tank Regulations - Transfer Flow, Inc. - Aftermarket Fuel Tank Systems
What is Legal