Sorry, I don't remember exactly which model of trailer you have (I could look it up, but, it really doesn't matter...
Most likely, if yours was / is like mine, you don't have a water filter installed. Mine came with a filter, what I initially thought was a spare, but, it turns out that it was the "original", just never installed
If that is the case, you can rule out the water filter...I would think you would have found it with all of your "exploration" if it is a replacement filter.
One thing that you can check...you've obviously got a restriction somewhere if both shore and pump water is a "trickle". If you don't have a water filter installed, try pulling the aerator / restrictor plate off one of the faucets. They screw on and have tiny holes in them to aerate the water and restrict the flow. If you, in fact, don't have a filter, it is possible that those holes have clogged up with sediment / minerals and that is causing the flow loss. Your pump should be providing 20-30 psi and without the aerator / restrictor on the faucet, you should get a reasonable flow from your pump. If you still don't get a flow, then you've either still got a valve thrown (not likely though; even with the winterizing valves thrown you should get water, that's how the antifreeze moves through the system) or a kink in the line near the water pump (again, not likely unless you've done some major pipe pulling). If you've been using your tank, and haven't been filtering the water going into the tank, my money is on clogged aerator / restrictors at the faucets...
I found it much easier to use an external (hose mounted) filter as opposed to the interior filter that was provided with the trailer. I used a whole house style filter (available at hardware stores) with a hanger that attached to the outside trailer wall at the water inlet. I also used it to filter water that went into the tank. Now that I've got the 5th wheel with the works in a "docking station", I've gone to the blue in-line hose filter, still an outside the trailer filter. Much easier to change and "keep an eye on".