Your concerns are justified IMO. When you build the 2x4 vertical supports under the A/C units, I would also brace diagonally left/right and fore/aft -- to the structure of the shell -- the same way that house movers do, when they move a gutted house. Is the sub-floor and framing strong enough to take the 2x4 towers and the bracing?
FYI the new search function in the blue box above [desktop version of this site] works great. Here are the "towing empty shell" results so you can check out the threads which have discussed this . . . there may even be some photos of the bracing:
Good luck, and please attach photos later, if you have a chance.
PS -- For one "look see" down the road upon which you may be embarking, "The Love Shack" thread might be a good read. The author has a great sense of humor, and has documented his work well:
"Eyes wide open" . . . that's all!
PS2 -- The trailer brakes must be operating, by law I believe, although some commercial haulers can probably avoid this requirement with a heavy duty tow vehicle?