There's another terminal with Signature and Arnold northeast of the airline terminals, towards 70. It's also right next to the control tower.
Signature Flight Support RDU - Raleigh-Durham International Airport
1775 E International Dr, Morrisville, NC 27560
(919) 840-2200
https://maps.app.goo.gl/QEmjKZrN8H3anz9w9