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I do have a possible solution for you but unfortunately there aren't any battery boxes available that will hold two 12V batteries and fit within a 21" length and 8" width. ...

The only thing that I have not included is a -down trailer hitch for my 2018 Thor Windsport My Shopping Cart Contents Blue Ox Base Plate Kit - Removable Arms - BLU77XR Towed Vehicle Battery Charge Wire Kit for Demco Supplemental Braking Systems - DM66VR Roadmaster 4-Diode Universal Wiring Kit for Towed Vehicles - 7-Wire to 6-Wire Straight Cord - RM-152-98146-7 Roadmaster Nighthawk All Terrain, Non-Binding Tow Bar for Blue Ox Base Plates - RV Mount - 8K - RM-677 Roadmaster Bent Electrical Socket Bracket - RM-910030-5 Brake Buddy Classic 3 Portable Supplemental Braking System - HM39494 |