Maintaining charge – 24V lead-acid battery from a 12V source

Hello everyone.

I’m looking for a way to use a PV-powered 12V lithium leisure battery to provide a continuous maintenance charge to the vehicle’s 24V lead-acid starter battery.
Is there already a ready-made solution for this? The Orion-Tr Smart DC-DC chargers don’t seem to be suitable for this application.
Many thanks