I have not done this. I am not familiar with your model so take this with a grain of salt. This will only work IF the hose is NOT clamped or zip tied anywhere along it's lenght. Looking at Keystone build videos it looks like the have not yet discovered clamps or zip ties. Anyway, as long as there are no clamps or zip ties to snag the hose it should slide through fairly easily.
If I was doing this...
Remove the basement walls and as much coroplast as you need for access. Disconnect the freshwater hose at the convenience center and the freshwater tank. Have a helper gently pull the hose to them while you watch it from the front. You are verifying that it is NOT clamped or zip tied.
IF it slides smoothly, attach the new hose to the old one as in the videos. Make sure there is a very smooth transition between the new one and the old one so it does not snag on anything. I would also attach weedeater string to it for future use (pull tape). Pull the old hose out carefully (spin it if necessary as you pull it).
Couple of videos demonstrating techniques
Keystone cougar underbelly
https://www.loveyourrv.com/inside-th...e-fifth-wheel/
Freshwater tank at 6.5 minutes.
Replacing thermostat wire. Good example for replacing a water hose.
https://youtu.be/NXkswK7TXuY
Wall fishing
Attaching fish tape
Keystone factory tour
https://youtu.be/cV0OZv0bCgc
Bottom chassis. Shows heated duct install @ 1min (down the middle).
https://youtu.be/j_1JQYSZbmY