Hey carvers2
I was up under my toyota Itasca working on a feed off the gas tank to my gen.
I noticed that, at least on my 1988, the tank had a flange in the top back for a level sender, a flange in the mid area for fuel pump acess that had a pressure fitting/hose for fuel out and a low pressure fitting/ hose for the fuel return line. Then there was a 90 deg nipple fitting foward of that that was welded to the tank top skin. The vent hose.
I was able to see all of this with out lowering the tank. The vent hose ran up to a nipple fitting at the upper end of the fill neck.
If this vent line gets a kink in it your vent is toast, or more likely, if it has a swag... a downward running length... fuel can get trapped in the swag and be slow to pass air... kinda like a vapor trap on a sink... this line should run with an upward angle its full length...
Ps I made stair step drive on ramps out of 2x6 stock cut in shorter lengths to three high with a stop chock 4x4 nailed at the end, to be able to get under the bugger.
Good luck