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I’ll borrow from @Mouseketab here:

” FYI, the easiest way to tell if you have a charge line without any meters. (This method does not tell you how efficient it is).
Turn off your T@B battery by disconnecting the negative, turning the battery cutoff, or removing the fuse at the battery.

Hook your camper to the 7-pin, crank the engine. Turn on an interior trailer light. If it comes on, you have a charge line. Turn off the engine, if the light goes out, you have an isolation relay. If the light stays on, you do not have an isolation relay.”

Our tow vehicle is an EV (Kia EV9.) The camper is equipped with a BB 100 a/h lithium battery and solar, both of which I monitor with the Victron Connect app. The main draw on the battery is of course the 2 way fridge, there's very little else drawing from it and when the fridge compressor kicks on, its draw can be seen on the app. We were on a trip and had stopped for lunch and I thought to check on the battery with the Victron app and saw that it was supplying almost nothing to the camper, even though the fridge was on at setting 5 for the trip. I then disconnected the camper from the car and instantly saw the lithium battery begin supplying the usual voltage that the fridge draws. I could only conclude that the 12V car battery probably supplies what power the camper demands as the fridge cycles, along with powering running and brake lights. I don't know whether the car is also constantly charging the lithium battery. I would hope that rooftop solar would remain active while traveling to do that job.

@DeeDee, assuming your T@B is wired similar to mine (2015-S) the charging circuit is pretty simple to trace. All the wires from the trailer pigtail run into a plastic junction box under the trailer, near the front.

The second post from the right connects (among other things) the charge wire from the pigtail to the positive wire from the battery. The first post from the right does the same for the negative wires. The rest of the spaghetti is irrelevant in this case.

Figure out which wire is which, and check both lines for continuity between the trailer plug and the junction box, and between the junction box and the battery. If both are intact and properly connected, your battery should be charging while you drive.

Looks like that fuse was it, it's charging now, woohoo! Thanks again @rfuss928

I happened to be out there right when the sun got high enough to cross the 5v threshold and could see it kick into charging mode in real time. Couple of screenshots just to illustrate, pretty cool. 

Right before it cut over (battery voltage +5v is the cutover threshold)
 


Charging starts once it hit +5v over battery voltage , solar voltage is drawn down immediately by the charger load and current starts flowing. 


Device list quick view in the Victron app:
 


Later in the day with the sun more overhead:




Not sure what's "normal" for a sunny day in September at 5,300 feet in Albuquerque but at least we're getting charge now

The .7 cu-ft Contoure microwaves used in the 320s are rated at less than 10 amps.

Did you check this fuse?  It is kind of hidden by the wiring and hard to see from above.  A small mirror may help...

@rfuss928 I did NOT check that one! And what do you know, it's blown!! Will have to wait for the sun to come up again to see if replacing it took care of things. I will post back, thanks!