I hope this helps: If yours is like mine, those valves are on the inside (mine are under the bathroom sink), but are difficult to spot. IIRC (sorry my trailer is not here), you pull the top upward to open, not rotate like many other valves.
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.