We also have a 2004 Marquis. We place a blue filter (get them at Walmart) on the end of the reel-out water hose, then attach the filter to the water faucet. If the water tank is empty, then it takes nearly an hour to fill the tank. I agree that by the time the shore water gets through the filter, then 40 feet of reel-out hose, then through the red hat valve and coach plumbing, there is not much water pressure at the shower and faucets. What we do is make sure the water tank is at least 1/3rd filled and turn on the water pump. The water pump will augment the water pressure up to an acceptable level.
If we will be parked for awhile or if we are in a bad-water area (Arizona, for example), then we attach our water-softener tank between the blue filter and the 40 foot reel-out water hose.