Put a 12V to 24V inverter just inside the firewall. This will isolate the “noisy” vehicle power system from the more fragile solar panel and power bank. But, it won’t be nearly as expensive as an inverter that can survive the engine bay.

The 24V output of the inverter should be wired in parallel with one or several 24V solar panels, then into the power bank input.

In this configuration the vehicle power system and solar system can charge the power bank simultaneously. No action is needed to change between power modes.

I highly recommend a Victron brand inverter.

Warning: Direct current (DC) systems present an extreme fire danger. Every wire must be the correct gauge, every fuse the right size, every crimp correct. No piece of the puzzle is difficult. But, one must have all of them to build a safe and reliable system.