[JellyBeanDriver]

How low is your 12V battery getting? Deep discharges are bad for batteries not designed for that kind of use.

A circuit like that can work but you'd need to beef it up to do any good.
Probably better and certainly less time to get a less smart car battery charger that doesn't care what the battery voltage is.

If you do want to use the Meanwell, you'll need to set it higher than +12V output not just due to the headroom of your circuit but also that lead acid batteries need to be charged higher than 12V (like 13.8) and occasionally need to be equalized by charging them to an even higher voltage for a bit.