I think you'd want to protect as much equipment as possible, thus before all electrical boxes and circuit breakers.
Another consideration is to mount it where you can see the status lights easily. We have some friends with a 5th wheel and their surge guard was mounted in a compartment behind a removable panel, making it a real PITA to diagnose a problem.
Progressive Industries makes a hard wired device with a remote readout if you haven't already bought the Surge Guard.
Denny, Connie & Shadow (former barn cat made good)
2000 Itasca Horizon, Cat 3126B, Freightliner Chassis