SUPCO "SPP6e" or 5-2-1 "CSR U1" both available on Amazon.
Most of us get the SUPCO and it wires across COM and HERM as you noted. SPP6e works better on RV than SPP6.
The CSR has three wires, two across COM and HERM. plus a third wire to another point. Comes with instructions. Supposed to perform better than 2-wire units, I have not tested it and don't recall anybody here using one.
EDIT BASED ON FRED's REPLY
Fred - SPP4e makes sense, and SUPCO recommends the series without "e" but I go with our tech friend Chris Bryant's endorsement of SPP6e.
OP - The non "e" SUPCO assist kits aren't "smart" like "e" series, the CSR, or an OEM relay setup.
