When you turn your ignition switch on. Do you get a green light at your 12v signal wire on the dc to dc charger.
The charger will not activate unless you have 12v at the signal port. Also I have found that the 12v source must be using the same ground as the 12v battery input feed

Have you checked to see if you have 12v at the signal wire going into the dc charger. If you have 12v at the signal going into the dc charger but the green light is not active try using a jumper wire from the 12v positive cable going into the 12v input on the dc charger and run it into the 12v signal connection on the dc charger. If the green light lights up it means your other 12v source is not in the same ground source as the your 12v positive input cables to your dc charger.
I have run 3 - 40 amp Renogy dc to dc chargers in my motor home and Jeep. I found that the 12v signal source has to have the same ground source as the 12v power input cables to work.
I also recommend installing a 12v on/off switch in your 12v signal power wire going into the dc charger so you can turn off dc charger when you don’t need to charge.

Thanks for the suggestion. It had been working fine, so I don't understand why it would all of a sudden stop working the way it is currently wired. However, I am game to try any logical suggestions. I'll try wiring it to the positive cable coming from the alternator.

Also, the green light no longer lights up when the trigger has power.

Thanks for the suggestions. I'll let you know how it turns out.

So if your green light from the 12v trigger source does not light up anymore would mean you don’t have 12v on your trigger wire. Good luck

Craig you are the Man! I would have spent forever trying to figure it out. I had similar issue as above and found your post and joined the forum because of it. Shared neg ground solved it for me. Thanks!

One question Craig if you are still monitoring, Why isn't the fuse panel ground the same ground as my battery ? I have the DC-DC hooked up to the battery pos and neg, and my D+ switch connected to one of the open fuse panel slots that runs from ignition. The fuse panel has a grounding bar, so I ran a switch as suggested with the neg to the fuse neg bar. If I run the D+ switch neg right to the battery no problem, but if I run to the fuse panel ground no bueno. Thoughts?

From what you are saying it sounds like the negative fuse bar is not grounded to the battery ground. Are you sure your on the 12v ground bar and not on the 120vac ground bar?
It’s better to be at the battery ground.
Good luck

Thanks! I'll check.

Fixed. Yep, wrong bus. Thanks again

Your welcome
Safe travels

I apologise in advance for not being aware but why do you need a d/c to d/c charger? I can see the sophisticated set up you have and realize that this is part of it but almost everyone has a batt bank and and a charger/converter already. TIA, Drew

Perfect timing of this thread. I also have a 20 amp Renogy that’s not working. I’ll do the same test. I’m on my way to Alaska and want it working for those extended rainy days

As for Drew’s question, we’re told the dc to dc charger is easier on the alternator and will provide the proper charging curve for batteries. I have 3 methods of charging my batteries when not hooked up to shore power, which is most of the time for us. In order of preference, solar, dc to dc, and generator. Most of the time solar is sufficient and I hate running a generator in a campground. On extended cloudy days, it’s a good option for charging batteries while traveling between campgrounds. I don’t need it most of the time but for $100, it’s the cheapest backup.

Make sure your 12v trigger wire is from the same 12v input source. 12v trigger wire uses the ground on the 12v input source to complete the circuit.
Good luck

Usually your inverter charger is low amp charger. That’s why people use a dc to dc charger. More amps going into your battery.

People use DC-DC chargers to charge the RV house battery bank using the vehicle's alternator. Say you have been boondocking somewhere for a while and leave with depleted batteries. If you drive for 4 hours to the next boondocking site and have say a 40 amps DC-DC charger you can add up to 160 amp-hours of capacity back into the house batteries.
The DC-DC charger also can boost voltage levels allowing for more power transfer even though the wire run may be long. Very handy for trailer owners. The input voltage after the long wire resistance may be down to say 12V but the charger output can still be at a high enough level to charge whatever the battery requires, say for example 14.2V for lithium. The DC-DC charge also adds isolation between the vehicle batteries and house batteries. Cheers, Ray

Thanks for that. I'm always learning!