I agree with Perry -- you have a leak somewhere. But it may not be where you are testing for leaks
-- it might be the regulator's diaphram is leaking, it might be the gauge itself, it could be the switchover valve, or a pinhole in the line somewhere.
Because both gauges slowly decline I don't think it is the gauge -- they would both have to leak. I would start by inspecting the switchover valve, the regulator, the rubber hose from the tanks, and then the rest of the lines. You can buy caps for the various fittings, so I would first disconnect and cap the line/hose running from the regulator to the trailer's lines. Test just that section to determine whether the leak is upstream of the cap (gauges still run down) or downstream (gauges don't run down).
Continue dividing the leaking portion of the system until you have isolated the piece that is leaking -- then you have conquered the problem!
Good luck and let us know how you fare.