The vehicle alternator neither knows nor cares about whats going on with the 7-pin, or the camper. It has totally separate voltage regulation goals, and limits.
The 12 gauge wire *might* be able to carry 30 amps, though that's rightfully more the realm of 10 gauge. 12 gauge is usually good for 20 amps, max.
More important, though, is the voltage drop over the length of that 12 gauge wire by the time it leaves the alternator and reaches the battery inside the camper. There's probably 20-30 feet of wire just in the truck side of things alone, and close to that much again inside the camper. Obviously that can vary a lot, depending on where the 7-pin connector is (what side of the truck vs. the alternator, front of bed, bumper, etc.) and where the camper plug is and how it's routed through the RV to the batteries.
Realistically, you're probably not getting a high enough voltage from the alternator to the camper battery via the 7-pin and the voltage losses to do jack as far as charging, unless the battery is halfways dead. At best it's a trickle charge, over a long (multiple hours) drive.
The setup I went with on ours was a Victron Orion Smart DC-DC charger as close to the camper battery as possible, with #6 AWG cable running down the same side of the truck under the cab and up into the engine bay, to a 50A CB then to the vehicle battery terminal. Used Anderson connectors in the truck bed for the connection. That setup gives a solid 30 amp charging off the engine/alternator with no adverse effects (truck does have the snow plow/camper package with 220A alternator).