I assume that you are not the first owner? Plumbing shown in the image strongly suggests someone has done considerable reworking of the water system. Galvanized pipe is rarely found in RVs. Also wonder what the fittings is on the shutoff valve on the bottom of the pic. That red valve looks like a gate valve, again rarely seen on RVs. Most are quarter turn ball valves.
Since opening the valve caused the pump to run its located on the pressurized side of the pump. How long did you allow the pump to run with this valve open? Did you look around to see if water was coming out somewhere? Inside or under the RV? If you leave it open and the pump runs for a while, several minutes, the water has to be going somewhere. Is there a pressure tank somewhere on the rig? I have a pressure tank with a shutoff valve so it does not fill with antifreeze while winterizing the RV.
Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & 2015 Mini Winnie 22R
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB