Kudos msiminoff IMHO yours is a model system.
The Essex type tower solenoid is superior to can type relays. For 100 ampere operation I would be tempted to go with a 200 ampere rated tower solenoid. Passing under the tow vehicle, encasing the conductor in heater hose or water hose is a smart idea. So is installing 100 ampere circuit breakers at BOTH ends of the installation. Using smaller than 2 gauge wire is IMHO a waste of time for tow vehicle recharging. Yes I do recommend 4 gauge jumper cables. They are SHORTER and can pass the same quality of amperage.
