Why don't you rig the tank with a low pressure guage and test it like plumbers check a house after the plumbing is complete. I just wonder how you determine if it leaks with duct tape and bent over hoses, seems to me they would leak. Just my thinking out loud at this point.
I also have an orginial 115 gal tank in my 1989 23WA that is probably close to needing replacing, but that would be a summer project.
My friend has a 350 V-8 w/4 barrel carb and he has had it for years and says no matter what he does, it gets 2 miles per gallon.