Hy,
if you are talking about a service box you can google truck body, utility box, body or service box or body. You have many manufacturers , Readings, Royal truck body( mine is a royal truck body 9 feet-had to cut the back of it to fit the camper). You could have them in steel ( 1100 Lbs mine), also aluminium and fiberglass. Also commercial truck trader sell used .
truck is heavier, but i have a lot of storage space, even more than i need. best modification i've did.had a good welder who helped me done the project.
good luck
Incognito