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!"

Solar panels attached to roof rack on Bob's own rig.

"Less wire running to charge controller, less voltage drop."
Better to mount on roof rack than directly to the roof. Fewer holes to drill in roof.

Put the black on the negative first, and put red on positive second. When removing the leads take the red off first, then the black.

The red is the 'live' so don't have the red on before you're grounded using the black.

Watts = Volts x Amps

Sounds like MPPT is the way to go.

Excellent trailer DIY conversion.

Bob Wells demos a simple solar system (missed the fuse?).

Really nice arrangement.