Solid state relays for DC are really expensive. If I could get dc solid state relays, that's what I'd use. They are basically a zero failure component. If you buy good mechanical relays, they are good for a lot of cycles. Me, for something like brakes on the truck, I'd probably pass on a relay for the truck lights. If you are planning on towing something lit up like a Christmas tree, run those trailer lines parallel on a relay. Since you are doing the lift gate thing, are you running an electric pump for the hydros's? You might consider splitting your load centers and putting one in the rear. It would save you a bunch of wire if you are running a large feed wire back there for a winch or whatever back there.
Just a thought.
Just a thought.