RV exchange

Hi,
I've been thinking about taking my family on a Road trip in the US for several years now. I've seen a few sites which offer RV swaps - but none of them look particularly lively and most require an upfront payment to list. I have a few properties in Spain I'd be interested in exchanging for a couple of months. Has anyone experience of exchanges and could give me some pointers? Is there a good portal for exchanges out there? Any information would be gratefully received.