There must be some way to drain the tank; at the worst, and very unlikely, you could drain it pretty well by pumping all the water out with the pump. However, there almost certainly is a valve or cap somewhere to gravity drain it.
As others have said, without substantial modifications, you're unlikely to be able to use the plumbing on your trailer in sub-freezing temperatures. It's unlikely that a little water in the tank would cause the tank to burst (it has room to expand, and the tanks are usually somewhat flexible), but water in exposed plumbing fittings very likely could cause them to burst. (PEX tubing itself is generally flexible enough to withstand freezing water without bursting.)
