Superb, detailed description of installing a solenoid into a vehicle, how it works, and why it's being used.
Use marine or automotive grade cable, won't breakdown due to vibration.

Interesting comment:
"That is the crude way of doing it.
A better way is to replace the solenoid with a CTEK D250S Dual. It's a really cool unit i have used for years.

https://amzn.to/2XtMFG3

First it's an isolator that dissconnects the starter battery from the house battery.
Second it's a DC/DC converter with chargecontroll. It compensate the voltagedrop and keeps full charging voltage.
When the housebattery is drained and that relay engages there will be a huge rush of Amps. This is how to avoid that.
Third, it has built in solar panel input with charge control so you don't need a separate controller.
Forth, when the house battery is full and you have surplus power comming in it will reverse and charge your starter battery.
It's an absolute gem of a unit. And if you can get your hands on one, go for it!"