Then the answer is "you are missing some parts".
The propane bottle should be mechanically clamped in place. When you grab hold of it and shake, the whole trailer should move.
Do some googleing on "Propane Rack" and see what is supposed to be there.
At the least, there should be a ring or a series of arcs to keep the tank from sliding around and some form of clamp to hold it down.
TT 2015 19RD "TheJayco"
TV 2003 F-350 "Montblanc" - Housebroken chore truck
Sitting in The Cheap Seats.
And proud of it!