The fifth wheel and truck camper will be an excellent source for parts. The windows from the 5er could work well also. You can study floor layouts for very small campers and truck campers for ideas. One thing that won't transfer well from the camper is the tub/shower because the tub takes to much room. Dad used a corner shower like this
Shop Ove Decors 76-in H x 34-in W x 34-in L Breeze Chrome Round Corner Shower Kit at Lowes.com in his last bus conversion. His had acrylic doors. This style works well because it still has plenty of room and you do not have to leave room for the door to open. You can pieces of board insulation to the bottom of the floor. It will be a bit tedious working under the truck around all of the wires and stuff but I think well worth the effort. Dad has always put lots of insulation in his bus conversions and brags about the ease of heating and cooling and his low electric bills.