Had my fridge replaced. Old was 3-way. Asked if I wanted to replace with same type. Answered yes because I wanted the ability to use 12V if necessary. After install going through instructions and installer said don't use with 12 V because your wiring isn't big enough. I know I don't have enough battery power. Solar/battery/inverter to be added soon. Question, in general, how difficult (which to me translates into $$$) is it to upgrade 12V wiring. If it makes any difference its. 92 Ford E-250 with high fiberglass top