IIRC, the Western Union splice is designed for wires that are both self-supporting and solid core.

Wires on the motorhomes should be neither - they should be stranded and be supported every couple of feet. Heat-shrink tubing resists flexing, the worst strain a double-J can have. The double-J also requires the least amount of wire to do - quite useful where there's little slack.

IMHO, any properly-soldered splice is better than using a wire nut.