used "C": how old is too old?

For me the nost important thing is the entire power train. If you would need an engine or transmission soon, that is a huge expense. Secondly all the appliances in an RV are quite expensive, so they must be checked out thoroughly. The tertiary thing would be any water damage, all the way from the fresh water tank to the grey and black water tanks. That means all the plumbing in between, too.

I wish you the best of luck, and happy camping.