Mine is 21ft also and it is the perfect length for me - I can use it as a second vehicle when needed, it fits in most campsites (i'm a camper not an RVer), and it is really easy to drive.
As Matt_Colie mentioned boat places can do cushions. Mattresses are more subjective and awkward as the sizes aren't always standard. The futon mattress is a good possibility.
One thing with the Dodge chassis is that they often had ignition module problems. The parts are cheap and small and it is a good idea to keep a couple spares on hand. Mine was so problematic that I ended up doing a mod and switching out to a GM module. No problems since.
Another thing to check is the propane system. Get the lines tested for leaks, etc., before lighting up the furnace and such.
I totally understand your "it's the one for me". I felt the same way when I saw my clipper - it just felt right. I did rebuild the engine last year and while it was out of service I started perusing newer RVs - they just lacked the personality.
One thing I tell new clipper owners is that there isn't a "snob" attitude about restoring them to original. Make the mods, upgrades, and changes that you want and make it yours.
Space is limited in these rigs, but there are some great ways to gain storage space easily without filling up your cabover bunk with totes of stuff. If you're interested, PM me and I'll give you some info on our clipper club and website - you may find the technical info helpful as you move forward.