Some where the wiring is crossed up, and the toad tail lights are being powered from a live wire.
Does the fuse blow if the wiring is plugged to the coach and not into the toad?
If so then the problem is probably in the coach end connector.
If not , check that the pins at the toad end of the wiring are powered up according to the installation instructions, before you plug in the toad.
If the, coach to toad wiring checks out, then disconnect any, added wires from the tail lights ( at the diodes if you have them).
If the fuse blows with the diodes removed then the problem is in the toad plug or the wiring from the , plug to the diodes.
Any repairs , or equipment instalation in the toad that might have compromised the wiring ?
Are you using a 5,6 or 7 wire hook up between the coach and toad ?
Is the toad set up with a battery charging circuit ?
A wired aux brake system ?