First.. I will assume your towed light connector is a flat 4 connector'
Four-Way Trailer Light Tester
Will test the output from the Motor home,, I have one similur to this (I have 2 like it) but the one I really like is an "in-line" it not only has the plug and LEDs like this one does but a socket as well so you plug it in and then plug the towed into it.
If you have output from the motor home TO the diode pack, and no output from the diode pack, to the tail lights, You have a bad diode pack. (Bad doide) you have two options.
1: External diode around it
2: Send it back and ask for a replacment.
If I recall correctly you said you have power TO the pack, but none coming out of the pack (Measure as close to the diode pack as possible.
One other possible problem, depending on the connectors under the dash where you break and make to install the diode pack (Break = Unplug , Make = plug in) you might have a bent pin on the diode pack's output socket.