[JRTJH]

There is a drawing on page 42 of the Keystone owner's manual that shows the plumbing from the sink shutoff valve, through the basement, out the side of the chassis rail, under the slide and up into the slide behind the refrigerator. There are two valves that need to be turned on (under the sink and beside the refrigerator) and one valve that needs to be turned off (low point drain, under the trailer) in order to provide water to the ice maker.

Make sure that there is water flowing through that PEX tubing and that the valves are correctly open/closed. That will probably resolve your water supply problem.