@Aifer , I guess no one actually answered your question above. But obviously you figured it out. The Victron would have worked, but was overkill since the EF already has a DC to DC charge controller built in. As you figured out, you only needed a LVD to keep your EF from draining the start battery. I assume you set the cutoff pretty high, so that the LVD would only close the relay when there was "alternator levels" of voltage available.
I've thought about the EF use case, and while it would be overkill a 800w inverter would be an interesting add for the truck (possibly go up to something like 1k-1.2k) that is directly run off the start battery. Then in a pinch you could charge the EF extremely fast rather than the semi-trickle charge of the 8amp DC to DC. You'd also have something that was capable of running a heavier duty tool or whatever in a pinch that the EF couldn't run. I'm not sure which one you have, my River Pro can only provide about 600w on the inverter, which is fine for most things, but probably not starting an electric saw or anything. If you have a Delta, then it would just be for the charging. This obviously complicates the install quite a bit, because you need something like the 4ga wire run to the back, verses the utilizing the big bonus of having a battery in the rear, is not needing to run a big cable between front and back. Or you'd have to have a setup where you pull the inverter out of a drawer when you need it and connect with short cable directly to the battery (not great if you want to drive while charging).
Sorry for the detour on this thread, back to the original scheduled programming.