I have owned about a dozen CNG cars and trucks.
The expiration dates on the tanks have been grossly understated.
The same tanks today have been given 25 years versus the 15 for older ones
If the deal is good enough you could try and find the parts to convert it back to diesel or diesel and cng.
I don't think a cng powered RV makes sense unless you stay local.
Most older buses had under a 200 mile range some only 100.
1998 Goshen Sentry Pusher 250hp mechanical cummins Allison MT643 w/side radiator (perfection)