I've been a RouterBoard user for several years now but never really used them for anything except basic routing. I have a setup with multiple sites doing a VPN back to a master site. I have all of the remote / master site routes setup correctly and I'm able to talk to devices at the remote sites from the master and vice versa. Now I'm trying to add another site connected back to the master for admin purposes and want to be able to access all of the remote sites through the master site without establishing direct VPNs to every site. Can someone walk me through setting the correct routes up?

I've attached a diagram to help explain what I'm trying to accomplish.

This worked for me. In the admin router:
1) add routes to all the remote router 10.10.x.x subnets using the vpn ip of the master router as the gateway.
2) add a srcnat masquerade with "out-interface=pptp-out" where pptp-out is the name of the vpn interface.

No guarantees. I don't have access to that network now, so I can't test anything.

edit: You can also add a route to the admin router in each remote router if you want to avoid using the masquerade.