It sounds to me like you have a water FLOW problem, rather than a water pressure problem.
In other words, the water is not getting into your water system quick enough to build up any pressure. You are using it quicker than it comes in.
The inexpensive water regulators many people use restrict water flow a lot. So do water filters. I would look to see if you have reasonable water flow without filter or regulator before digging into the trailer.
If you do have good flow, it could be tough finding the restriction in the trailer system. Or it could be as simple as a clogged water restrictor on the shower hose.