DC-DC Charging ?

Put the dc to DC charger in the existing "house" charging circuit on the "house" side of the isolation solenoid.

While in use--there is no back feed path.

Then when the truck is shut off, the dc to DC will be isolated from the chassis battery.

Thanks, interesting.

My question was more about the Renogy 20 amper with input from starter and output to house. After some more reading of its manual, it says: (after saying the input and output negs are all on the same chassis for grounding) for the positives:

"the DC-DC output will connect to the 12V auxiliary or house battery that you are intending to
charge. These batteries may be a different chemistry from the starter battery. The DC-DC input
and output terminals are isolated, meaning that the output voltage can be kept stable without
interference from the input circuit. This ensures stable and correct charging of auxiliary batteries. It is best to place the DC-DC closer to the battery you will be charging primarily."

I don't want my SiO2s in parallel with the FLA starter although I am told that is not a problem when floating or charging them. So right now in the TC I have disconnected the 7-pin charging to the camper batts.

I would like to have "alternator charging" as a back up to the solar, since we can't carry a portable gen along in the TC set-up.

I was thinking of using the 7-pin pos and neg in the camper that normally go on the house batt terminals as the inputs to the Renogy and output to the battery. Not too worried about the amps that would supply and the truck has a 40 amp fuse for the pin 4 charging wire so that would cover the higher draw from input than max output of 20 amps. I think.

So would that work like it says and not put my SiO2s in nparallel with the starter batt?

https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q5VYPCF/ref=sr_1_3?crid=24ISMNS5CEXIC&dchild=1&keywords=renogy+20a+dc+to+dc+charger&qid=1613847979&sprefix=Renogy+20a%2Caps%2C399&sr=8-3

EDIT--could I get the same thing by just using a solar controller with the 7-pin pos and neg into the array side?

Parallel would imply bi-directional flow and that is not really the case. The DC-DC just allows power in one direction. Then it is just a matter of how the DC-DC is connected to the truck and if that connection has any isolation.

There’s more than one way to configure it, but for the most part it’s a parallel scenario, and often* necessary to protect the alternator when using LiFePo4 batteries - Why?? Because of (if one dare believe Victron, et al...) LFP’s high current receptivity...Also, for newer vehicles, the dc to dc converter needs to be ‘Smart Alternator’ or Euro 5&6 compatible...

*depending on components and load

Here’s an interesting primer on the subject:

https://www.youtube.com/watch?v=dzU_pQFa19s&feature=youtu.be

3 tons