I remember times when 20lb propane cylinders had no safety valves and in the past I have seen lot of houses where cylinders were kept under kitchen sink.
So I don't think it is big deal to carry them in cargo trailer tilted, however for liability sake, I would make sure the tilt keeps the safety valves exposed to vapor not to liquid.
Cylinders have > 20% of vapor space above the liquid, so it will take good tilt to cover the valves with liquid, but I have never seen official publication listing the tilt value.
Easy test to do - insert a fitting on cylinder valve to disengage push valve and open the flow slightly. If the vapor comes out - you are good.
If liquid, you might want to change the angle.
Still making sure you have no open flame under fridge during driving would be good idea.
Than I see propane copper tubing going next to 120V cable and that's above the fender, where flat tire might torn it apart.
We can beat the horse endlessly.
