12v parallel batteries
Two things. If your truck wiring is 12V and you intend to charge the batteries off the truck, you have to connect them in parallel for the same reason your now-knackered 6V batteries were connected in series: to be compatible with the truck 12V charging system.

Also, your inverter probably needs 12V, though it might support 24. Regardless, the truck only provides 12V (I assume) so that dictates how you connect the batteries.

To charge them, you should connect the parallel-tied batteries to the main battery in your vehicle, with a self-resetting fuse and a disconnect.