My situation is unique with the camper, but I’ve been running dual batteries with a simple 100amp continuous duty solenoid for years. Zero issues. Super simple.
The solenoid is fed from a three position rocker switch. That switch is fed power from the main battery via the ign switch and the other is from the aux battery. With the rocker in the main setting the solenoid joins both batteries once the ign is on. When the ign is off the solenoid disconnects and will allow the aux battery to discharge without effecting the main cranking battery.
Middle position kills all power to the solenoid and the truck runs on the main battery alone and the aux battery won’t charge while running. The third position allows the aux battery to energize the solenoid on the off chance the main cranking battery is dead.
My camper has a good amount of electrical load when in camp mode. Lights, 12v fridge and charging stuff will put a solid dent in a fully charged battery overnight.
I do have a solid state ASR relay to replace the the solenoid with for the eventual move to add solar. This way when the solar is putting out more than 13v it will join the batteries and charge both.
Thats just the way I do mine. But it’s simple and very reliable.