As dlhilliard noted, most water filters are prior to the water pump in the smaller travel trailers...some have one located under the sink for filtering kitchen sink water, but, most are set up assuming that the water in your fresh water tank would not need to be filtered...you either filter it prior to putting in your tank (using the filter attached to a hose as opposed to your plumbing) or you don't need to filter it to begin with. The filter "installed" prior to your pump is more for campground water as opposed to the water in your tank. The filter attachment in our '08 210WBS is in the city water inlet line, after the check valve and before the pump connection, in the middle of a rats nest of pipes, valves (winterizing bypass) and wires in a crowded cubby hole under the television shelves.
I mainly rely on a whole house type water filter mounted outside the trailer for campground water hook ups and when I use the fresh water tank I normally use that same filter for filling the fresh water tank. I have a Pur water filter on the kitchen faucet for filtering the water for drinking as a "final defense" on crud and taste, if needed. The outside filter and the Pur water filter on the faucet are much easier to access for checking on the condition of and the changing of the filter elements than the under cabinet or "tucked away by the pump" type filters. I filter more for removal of debris, some campgrounds can have wells that carry a bit of sand and other grit in the water and I like to get that out to eliminate wear and tear on the faucet washers and to prevent sediment in the hot water tank. I haven't found any water in our travels that wasn't drinkable and the Pur filter on the kitchen sink is more for "just in case" than any real need...we don't filter the bathroom sink faucet (aside from the whole house sediment filter mounted outside) and I've never found any water that wasn't drinkable. Some taste better than others, but, all of it was drinkable, although, truth be known, I don't drink that much water :wink:
My recommendation would be to use a hose mounted filter to filter the water prior to filling your tank or attached between the shore faucet and your city water connection, or use "point of use" filters at the faucets to filter your tank's water. Easier for inspection and changing of the filters when needed as opposed to gaining access to it in that cubby hole.
