Hey,
Yeah, you are right, they seem to be rare all of a sudden.
However, I did a little research for you with what I have on hand. Almost all my 'R's are short frame units, however, I do have a dump & a cement mixer, they are the long chassis. It just so happens that I have a first run 'B' & the new Ford wreckers, the wrecker bodies are the same mold & are spaced perfectly to fit the long chassis Mack 'R' !! So, get any 'R' & any wrecker & modify them together, all is good, no? I did a batch of Ford L9K's in a snow plow conversion only to find (much later!) I had a bunch of long chassis stashed away. Well the effort makes me appreciate them now. heh heh..
Watch the auction sites, & hit up your LHS & ask friends to also in their/other areas. They are out there for you. Also, check out Plano PE accessories, mirrors - steps, etc.
Hope this helps a bit..
I must of been sleeping when Athearns R Mack trucks came out. I went looking for the tractor/trailers and tow trucks. Apparently they are no longer on earth. I hope Athearn does another run of these. If anyone knows where I can get some of these please let me know. I can only find the railroad logo dump trucks.