Very Poor Gas Mileage on 1997 Chevy Silverado

I would do the usual tune-up stuff first.. plugs, wires, cap, rotor, filters, PCV valve, etc. Try to avoid aftermarket parts if possible. You may pay a little more for OEM GM / ACDELCO parts, but my past experience showed me it's not worth the aggravation.

I'm guessing this is a TBI. The throttle body may be getting gummed up. This is a pretty common issue with the 4.3 & 5.0 / 5.7's. Even the thirsty big blocks were prone to this. Pick yourself up a base gasket while you're at it, and a can of "Berkible 2+2 gum cutter". Wear rubber gloves when using this stuff (don't worry, it's "sensor safe"). Remove the throttle body and when you turn it over, you'll be surprised at the carbon build-up in all of the passages, etc, and how much better it will run afterwards.

Check for cracked / dry rotted vacuum lines too while you have it all apart.

I'm actually surprised you were getting the mileage you were (8-12), considering it's a 350, and the weight of the camper.