Even a non-mobile greenhouse needs to be anchored, so it would seem easy enough to screw or bolt the foundation to the trailer.
If it is to be a hoop style or a variation using pipe, you can use half round conduit clamps with ears (you might need to look in the electrical section of a big box store for conduit clamps), attached to the sides of the trailer. Drilling a hole in the curved part of the clamp and screwing into the pipe will keep the pipe from popping out if the trailer is moved.
If on the other hand it is a "hard wall" greenhouse, there should be some sort of base which could be anchored to the floor or sides of the trailer
I seriously doubt you would want to take it on the road at highway speeds due to the wind load (if there is no wind and you are traveling 50 mph, you have a 50 mph wind load). IF you are considering taking it to shows or some such activity obviously it would have to be built very sturdily. Remember even if it will just be moved around a campus or field that the plants need to be secured so you don't have casualties when you hit a bump. Also some 2-4" Styrofoam insulation under the bed of the trailer would certainly be useful during the winter since wind can get under it and add to the cold of the sides and roof.
