I have the same trailer. You can access the water pump by opening the little door below the closet that has the heat vent in it. This door is hinged, open and remove the duct attached. THEN, go under the fridge and open the door to the small compartment. You can remove the back of this little compartment without much trouble. Now you can see much better.
Find the strainer in the clear plastic holder, unscrew it and remove the filter. Clean out the faucet aerators too. If things improve at the faucets but not at the shower, you might have to replace the moen cartridge. They can get messed up over time. If this 25FB is original you might have to replace the cartridge.