If it's a slight dimming, it's probably normal, caused by the length and gauge of wire between the lighting controls at the front of the motor home and the lights and toad connection at the rear.
Powering both the toad lights and the rear lights on the motorhome increases the load on the rear light wires by 50-100%, increasing the voltage lost along the wire a like amount.
If you have diodes or combiners (to convert separate turn signal and stop lights to a single bulb in the toad), they'll also add about 0.3 to 0.5 volts of loss going through them.
If you're losing a major amount of brightness, I'd look for corroded connections between the switches at the front of the motorhome and the connections in the rear.
A quick way to solve the problem is to switch from incandescent bulbs to LED replacements in the toad's taillights. The dimming is proportional to the amount of current flowing through the wires, and the LED lights use such a small amount of current compared to conventional bulbs that the additional load becomes insignificant.
