In my old bus I used a cheap solenoid wired through the ignition switch. When the ignition was on/ motor running the crank and the house batteries were connected and charged from the alternator. As soon as the ignition was turned off the batteries were isolated and only the house batteries were drained.

The amount of extra power you get from the fancy setup is minimal and the money better spent on extra battery or better batteries.

One of the beauties of lithium batteries is that they charge faster as there is no absorb and float and you can drain them a lot more with no harm 20% is recommended but lower if needed and you still have full voltage and they never need to be brought back to 100% SOC (full charge) and they are expected to last at 8 - 13 years but as far as we know none have every died yet through use so life expectancy is not fully known.

1/3 the weight and size and you can drain and charge as fast as you like. I pull 60A @ 24v off mine regularly using an aircon and water heater as well as the normal household vacuum cleaner and I run a 240v fridge 212 ltr capacity.

Hope the 120w solar is not too heavy or is it 2 x 60w giving 120w?

For the solar try and get a decent plug installed to connect direct to your batteries so you can just plug in with a decent connection. Dont use a cigerette plug as there is too much resistance. Ither the special plug with return fingers all around or better still a dedicated anson plug. Aslo the wires between the panels and the plug should be a large as possible, most of the ones sold are realy too small but see how it goes and then get thicker cables if you think you are not getting the charge you expect.

Remember with AGM's etc when you go to absorb mode you are only about 80% charged and the last 15-20% takes a long time.

Hope you enjoy your solar and it makes your travels better.

Seems like you have the right mind for travel. Find the spot and stay till YOU want to move and then just to the next great spot.

Regards
Brian