I have a fuel slip tank in my current truck and that can weigh up to 500lbs when full and sometimes but a atv in the box also so there would be close too 1000lb in the box
Whether 500 or 1000 is a lot or a little depends on the truck and its rated capacity (with the air springs). A one-ton truck shouldn't sag with that sort of load, but a half ton might be near bottomed-out.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC