'What can you fill a barrel with to make it weigh less/'

Not much to wrap your head around. Stronger springs will hold up the load better. Your E tires are good for whatever capacity you have them aired up to.
Rims, lugs, axle, spring mounts may be the next weakest link.
You have 2 choices. First, you’re not really out of the realm of reality with your older? Tundra and little popup.
I think that’s what you said you have.
Second, or just get a heavier duty truck and then you won’t have to think about or pontificate or worry about any capacity with a popup.