This is one of the key differences between using one of the official RV vendors (like Meachams) versus an un-official one. Official RV vendors are listed on the Fort "Amenities" website tab (scroll down to "RV Rentals":
https://disneyworld.disney.go.com/resorts/campsites-at-fort-wilderness-resort/services-amenities/
Meachams (approved) only needs your reservation number and they work directly with the Fort/Disney to find out your site number, Disney gives them access to the campsite to set up before you arrive, so
@dizneygirl55 can get her notification text and drive in and it's all there ready for them.
Nothing against Kissimmee Orlando RV Rental but as an unofficial vendor they need you the guest to do more work to allow them entrance to the Fort (via your call to the Reservation line to add their name to your reservation it sounds like) to get in and set up for you. So yes you would have to call.
It's sort of like renting a golf cart from Disney (their approved golf cart vendor is themselves) or renting one from an off-site vendor. I've no doubt that Meachams pays Disney for the privilege of being an approved vendor and that cost gets passed on to the guest who can decide if it's worth it or not. Off-site vendors are cheaper in the golf cart rental world but require more work on the part of the guest to pick up and return. That's a simple example and campers are more complex with features and add-ons but the principle still applies.
So for the OP, yes you would have to call and add them to your reservation (as you would if an extra person all-of-a-sudden wanted to stay at FW with you after you'd already made the reservation). Being on the reservation grants them legal access to your campsite. If you are not legal or on a reservation (an Uber driver for example), you're restricted to the Outpost/Checkin area and they can't go back to the campsite. Supposedly.
Bama Ed