They're giberish, more or less, used to decribe the floorplan. Combinations I have seen are
BH bunkhouse
RK rear kitchen
FK front kitchen
SS super slide (meaning one large slide)
BS bedroom slide
RL rear living room
RB rear bedroom
So if we pull a bunch of these out of a hat to make an imaginary trailer
29RKBS would be 29 feet, with the kitchen in the rear and a bedroom slide out
26FKSS would be 26 feet front kitchen with a super slide
So you get the picture? My 5th wheel is a Rockwood 8280SS, so all that says is a 28ft with a super slide. If you wanted to know more you'd have to go to the Rockwood site or ask for a tour.