Probably not much help, but tracing the water lines back to where they split off from the supply lines--if there is a clamp, you can take the joint apart and see if water will pump there. But, easier said than done with the way they hide things now, and use PEX lines/clamps.
I still think the plug is in the shower head.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired