Since you are going to buy a vehicle specifically to be towed, then why not simply get a vehicle that can be towed four down? As already suggested, a Jeep Wrangler makes a great toad. You aren't just adding a third vehicle to tow, you are adding a versatile vehicle that can take you to many off road destinations while out in the RV.
Or instead of buying a third vehicle at all, you can always get a car trailer too and use the vehicles you already have. The problems with adding a car trailer to the mix are obvious, but lots of people do it.