If you have a battery connected to a light bulb it is on. If you want to turn it off you need to add a switch to open the circuit. There are two wires. The red wire connects the battery plus to the light and the black wire connects the battery minus to the light. To turn off the light you can cut either wire and install a switch.
My cheap solar controller has the battery plus connected to the solar panel plus all the time. The battery minus connects to the panel through the switch. The switch in the controller is full on for bulk and switching on and off for PWM voltage limiting. The switch is in the minus side because transistors, that are NPN or N-channel are faster, better, cheaper, lower resistance, than the PNP or P-channel alternatives.
Renogy has a controller that switches the plus side. I sent an email with the question: can I connect the solar panel minus side direct to the roof to use the steel roof as the minus conductor saving money on the wire? The response was that it could work but was not recommended.
I have a cheap amp hour meter. I connect it to various places depending on what I want to measure. When not somewhere else I put it between the solar panel and the controller. The amp hours passed through it charging tells me how much I used last night as well as how it is all working today. I have used it with my MaxxAir fan. Works fine.
In the original post you say you have a positive ground system. Is your house battery plus terminal connected to the vehicle chassis? If all the house wiring is isolated from the vehicle wiring it doesn't matter. If the two come together bad things could happen.