we almost bought a Ridgeline for a tow vehicle. even though we loved it, it was soon evident that it was a bad choice for towing a travel trailer.
the 5000lbs. tow capacity is really for boats, not for pulling the flat front of a travel trailer through the air.
add that Honda is the only truck manufacturer that recommends NOT to use a WDH on their truck.
this goes against every other truck manufacturers's recommendations for towing.
this tells me that Honda doesn't really consider the Ridgeline as a true tow vehicle.
you'll have to decide whether or not to use a WDH. check out the Ridgeline Owners forum, there's a lot of them using WDH's to tow with.
the 21RS model was available from 2006-2012. up to 2010, the front had bunks but i think that in 2011, they pulled out the bunks and put a front kitchen in their place, till they dropped it from their line after the 2012 model year.
i don't think they sold a lot of the 21RS models, at least no where near as many as they did their hybrid models. so there may not be many out there to find for sale.
here's a 2009 brochure:
shows the fictional "dry" weight of 3559lbs. when it left the factory, it usually will weigh about 400lbs. over the brochure "dry" weight.
so that means it'll weigh around 4000lbs. BEFORE you add the battery, propane, water and cargo.
but your biggest problem is the 21RS's hitch weight.
with a base/dry hitch weight of 594lbs., it'll easily be over the Ridgeline's max hitch capacity when loaded.
and Zoe the Wonder Dog(R.I.P.)
'07 Roo 23SS pushing an '07 Chevy Avalanche
Equalizer WDH and Prodigy BC