If the "new" rca was hired to be primary on the Aux., then yes you have a valid grievance due to your pm not following the "matrix".If that is the case why is he learning the other route? The matrix is supposed to be established in every rural office and each carrier (after they have become proficient on their primary route) are to chose 2 other routes to work as secondary or one secondary and one tertiary.
If you have chosen to work the aux. as a 5&1 and not a 6&0, which is what you have had to have chosen to be working the Aux. and another primary route, then the new Rca should have worked the aux. You can grieve for the difference in $ you received verses the amount you would have received if you worked your primary.
Find form 8191 in the knowledge base and fill out the top section to 3.C. If your pm rejects your request for pay in a (1st step) discussion you have with him, then hand him the 8191 and have him initial in the 3.B spot signifying he agrees that was the date you filed. The complaint should be asked in the form of a question. Something like, "Did management fail to follow the contract when I wasn't worked on my primary route on (date)?". Keep the original and give your pm a copy. Call your steward or ADR/Dr, which you can find off of the NRLCA website, by putting in your zip code. Good luck.