Too little information to really provide anything other than generic guidance. You need a DC Amps clamp meter as well as volt meter and need to work you way around the system methodically.
You have 400Ah so low voltage every few hours sounds like a high drain and should be easy to find a cable with current flow when it should not be. You will also have something warm.
The only thing of note is the trickle charge from lithium to lead acid. Read my FAQ on how the trickle charger works. With lithium, due yo the higher voltage there will always be a drain on the lithium batteries to the starter. If the starter battery is old the trickle charge may be quite high or if this is a van with a high system current. Is the starter battery warm. What happens if you disconnect the trickle charger (pull the fuse).