My new 2015 Jayco 27RLS has this same uphill tubing to fill the fresh water tank. My fresh water tank capacity is 80 gallons. I did not find out about the uphill gravity fill tunnel until I was preparing to sanitize the fresh water system for the season. I made a simple solution to this problem.
I clamped 40 inches (doesn’t have to be exact) of clear vinyl tubing to the end of a clean funnel. I made sure the clamp was snug but not over tight. I can now feed approximately 19” to 20” of the open end of the clear tubing (the end not clamped to the end of the funnel) into the trailer gravity fill tube. This gives me about 20” of tubing left outside of the opening (from the gravity fill opening to the end of the funnel). This isn’t an exact measurement; one can make it shorter, just make sure you have enough tubing outside the trailer gravity fill to create good gravity pressure and enough tubing inside the gravity fill opening to go up over the incline in the trailer gravity fill hole. Hold the funnel as high as the tubing will reach above the opening of the gravity fill hole. Poor the solution of Clorox and water (I use 1/4 cup Clorox to one gallon of water as recommended) into the funnel and allow it to drain through the tubing into the gravity fill opening. Gravity will empty the contents of the funnel down into the tubing, up and over the trailer's gravity fresh water entrance tubing and into the fresh water tank. I repeat pouring the solution of Clorox and water into the fresh water tank until I have finished adding the correct amount of solution into the fresh water tank. There will be a small amount of solution in the clear vinyl tubing at the end of the process. I have a funnel that I can unscrew from the tubing and gently blow the remaining solution out of the vinyl tubing into the fresh water tank. If you can't unscrew your funnel you will probably spill a small amount of the solution at the end of the process. This would be the small amount remaining in the tub and the end. Most experts’ instructions and my trailer manual say to add one gallon of this solution for each 15 gallons of fresh water tank capacity. For example, if your fresh water tank is 30 gallons you would add two gallons of the solution to the fresh water tank for sanitizing purposes (solution is the combined ¼ cup Clorox to a gallon of water). Then you would finish filling your fresh water tank and follow the rest of the water system sanitizing process from this point to properly sanitize the fresh water system.
To simply add fresh water to through the trailers gravity fill opening into the trailers fresh water tank, one can use a special adapter you can buy at a RV supply store. This can cause some water back flow and spillage when the gravity opening goes uphill at the opening. Or do as I did and create a special adapter that I think does a better job of filling these types of gravity fill openings (openings with the fresh water fill tubing that goes up from the gravity fill opening before turning down going into the fresh water tank). To make my own adapter I clamped (snugly but not over tighten) a barbed garden hose connection fitting that has a 3/4'’ female garden hose end into one end of 19” to 20” of clear vinyl tubing. I can now attach the ¾” garden hose end to my white water supply hose and fill the fresh water tank. I feed the open end of the clear tubing “up and then down” the gravity fill tubing. I turn on the water and fill the fresh water tank. I can use this same adapter any time I want to fill my fresh water tank.
I purchased all the materials to make the funnel tool and the fill adapter from Home Depot. I’m sure you can also find these materials at other similar retail home supply stores. The supplies that I purchased: my funnel end was ½” so I purchased clear vinyl tubing that was 5/8” OD and ½” ID (OD = outside diameter and ID = inside diameter). I used ¼” to 5/8 “stainless steel clamps (you can use a size larger 3/8” to 7/8” if that works better for you). I used ¾ x ½” brass adapter fitting that is barbed.