A modular electrical distribution system for an RV comprises a water-tight housing for mounting to the RV with a plurality of water-tight openings for receiving electrical cables or wires from a towing vehicle and/or auxiliary electrical devices. The system includes a power-in assembly, a trailer connection assembly and a breaker assembly that can be tailored to the particular electrical distribution needs of the RV. The trailer connection assembly includes a plurality of electrical terminals for engagement to wires connected to electrical components of the towing vehicle, such as brake lights, turn signal lights, etc. The breaker assembly includes a plurality of circuit breakers or fuses connected to the power-in assembly and to a plurality of corresponding output bars with output terminals to connect to the cables/wires for the auxiliary electrical devices. The system can include an auxiliary circuit for connection to an auxiliary power source, such as a solar panel array.