You did the ammeter bypass correctly. You are right that you need to leave both cables going into the cabin, since that is the only way to power the cabin. Actually, you probably need just the ALT cable since all loads come off that. The BATT cable is in series with the ammeter, to measure all flow into or out of the battery to the cabin, the only exception being special accessories like spot-lights. For hard-core electronics types, I pioneered a way to keep your ammeter working yet start bypassing it at higher currents (search).
