Truth be told, the charge is minimal at best. It’s a strong maybe. Depending on what all draws are going on in the camper – the fridge, an inverter, and other parasitic draws- you may get a little charge or you may actually be discharging the batteries while driving.
The reason is the wires to the 7-pin connector are so small and it’s such a long distance from the truck’s alternator/battery to the trailer’s battery.
If you really want a decent charge from the truck to the trailer, look at installing a DC-to-DC charger separate from the 7-pin connector:
For instance, I’ve measured it and depending on what’s running in the camper, it’s either 3-5 amps or actually running at a deficit. Here’s what I saw after leaving my first camper sit for a week with the fridge running + other parasitic draws. I plugged in the truck and opened up my battery monitor. You can see there is a 3.7 amp push of power into the battery.
In another instance, I’ve switched rigs with more 12v draws going on and I installed an inverter that powers most of the outlets in the RV. Between the inverter being on, the bigger double-door Norcold fridge, and other random 12v parasitic draws, I was running at a negative draw on the batteries for this particular drive.