At this point, it's just cargo. Four big 10K pound, 4" wide straps should be enough. You might put some long 2x4's under the straps on the edges of the roof of the camper to spread the loading. Wind resistance will be the only worry. For a belt-and-suspenders version, you might put a smaller strap from the rear tie downs inside the truck bed around the back of the TC and to the other side, just to make sure the thing doesn't slide out the back. You just don't want that box to sail off to the side of the roadway.
jefe
PS: tubapete, what tuba do you play? BBb, CC, DD, Eb, F, G, Bb tenor tuba? My old buddie Roger Bobo owned and played all of these in the orchestra.
'01.5 Dodge 2500 4x4, CTD, Qcab, SB, NV5600, 241HD, 4.10's, Dana 70/TruTrac; Dana 80/ TruTrac, Spintec hub conversion, H.D. susp, 315/75R16's on 7.5" and 10" wide steel wheels, Vulcan big line, Warn M15K winch '98 Lance Lite 165s, 8' 6" X-cab, 200w Solar