David, Lets see if I've got this right, your RV has only red lights, the brake light/turn signal filament, and the taillight filament in the same bulb. Correct so far?
Your towed has a single filament brake light bulb. Correct here too?
And the amber light has a filament for the turn signal and one for the tail lights, correct?
Now, lets imagine what would happen if you connected the left brake/turn signal wire from the RV to the left brake light wire for the towed and you installed a blocking diode in the line between that connection and the towed wiring. Now the power from the RV cannot flow into the towed or to the right side brake light.
Now you have a power source for the left brake and turn signal coming to your towed. If you step on the RV brakes the brake light on the towed comes on, so far so good. Now you turn on the RV's left turn signal and the towed's brake light is now your towed's left turn light. Nothing in the MVC says that it has to be the yellow light on the towed.
Now do the same thing to the right side. Disconnect the cable from the RV to the towed and everything works normally in the towed. Simple right?
There is absolutely no reason to buy and install the single lamp to dual lamp converter.
Now all you have to do is connect the parking light wire from the cable to the parking light circuit of the towed with the appropriate blocking diode and you're legally wired to tow.
