In my RV, I have my Starter battery running to a BlueSea Relay then connected to my Aux batteries. The current Relay only connects the two batteries when car is on and Alternator is charging. However, I want to install trickle solar chargers so my Start battery never dies. Can I replace the BlueSea relay with a Victron DC-DC 12|12 Non-Isolated charger so that the Solar will charge the Starter batteries, which will run through the Victron, which will keep the Aux batteries full?
if the Blue Sea relay is an automatic charge relay (ACR), these are voltage sensing - like the Cyrix.
In this case, if you charge the start battery with an external charger, then at ~13.8V, the ACR will close, and both batteries will be charged.