I would start with a heavy duty trailer that already has a box on it.
A commercial 40' semi van trailer would do fine. They can be picked up used very cheaply.
A reefer trailer would be better, since it's already fully insulated and has an awesome refrigeration unit on it too!
Of course, you will need a truck with air brake system to pull a semi trailer. I wouldn't want to pull a 40' heavy duty toy hauler with anything less than a class 6/7 commercial truck anyways (F650/750, Freightliner M2, International 4000-series).