You have a slight leak in the propane system. The reason the gauge drops off all of a sudden is that the high pressure in the two pigtails is feeding the regulator. When that reaches the reg cutoff point the pressure is gone. The only real way to find is to cap all appliances and verify the trailer gas lines themselves. If all is good reconnect one at a time. If you use dish soap as a leak check wash it off any copper or brass when done as it corrodes them.
Just as a shot in the dark, does your oven have a pilot setting on the control. And is it off?
Art.