Gas tank replacement/upgrade

100 gallons would be on the smallish side for most diesel coaches and not at all unusual on a gas coach. He also mentions a conversion to ethanol which I can't imagine being done on a diesel engine.
I've been wrong before, however!

You don't give much information but at 100 gallons, I might guess that it is a diesel engine. I don't know of any gas coaches that had a 100 gallon tank. Not saying there aren't some out there but I have never heard of them. I know you said ethanol use but just checking.
If it is diesel, I'd drain the tank and just try new diesel with an additive to kill any fungus. You may also want to replace your fuel filters.
If it is gas, drop the tank and get it flushed and coated. Many radiator shops will do this for you.

I'd inspect before I replaced, but to each their own. May be a perfectly fine tank. If the tank doesn't have rust and if it were me I'd just siphon the gas, add fresh gas, marvel mystery oil in the gas, and a bit of marvel mystery oil to the oil. May be worth adding a drip of oil down each spark plug hole (when the engine starts it WILL smoke), but this is to lubricate the cylinder a bit.

I had my tank removed by a Winnebago Dealership several years ago. The Dealership removed the fuel tank bands, used a transmission jack to lower the tank then siphoned out the fuel to inspect the tank.