The R-21 classics don't really need a huge tow vehicle with a super duty power train. Mine, with trailer, weighs in at around 4500 lbs and towed very nicely behind my "03 Chevy Astro (pics in my gallery) from Salinas Ca to Lewiston ID through temps as high as 110 deg. at 75mph with the AC on, with no issues what so ever. It had the factory tow package (rated at 6000 lbs up to 1000 lb toungue wt) and I put in an extra HD trans cooler. I currently pull it behind an "01 Chevy s-10 Blazer(same eng/trans as the van but better day-to-day mileage around town). Neither rig had/has any problem getting in to or out of the valley here (a real big climb). Braking has never been a problem with either vehicle.
Given the easy towing of the R-21 and it's minimal accomodations I think it would also tow very nicely behind a small motor home (Mini-bago or fullsize conversion van). It will certainly tow well behind any full-size pickup like an f-150 or silverado or Ram. A V8 would maybe give better gasmileage while towing, but if it is going to be an everyday driver a big 6 will have enough power without eating you out of house and home.
Of course, if you intend to move up to a 25,27,29 in a year or three, you may want to go with a much bigger tow vehicle in anticipation of that future boat's needs. The inter-spousal discussion of "moving up" is much easier if it doesn't involve a new truck too.