Campus Manager Data Collection will only manage the devices which are in the Device Credential Repository and have not been filtered in the Data Collection filters. First, you need to check DCR to make sure all of the devices on your network have been discovered properly. Campus Device Discovery only updates DCR with reachable devices. So while you have have a lot of devices with CDP enabled, if they do not have SNMP enabled, or the SNMP credentials you configured for these devices in your Discovery settings is wrong, then they will not be properly discovered.

Another thing that could affect what is seen in the topology map is ACS integration. If LMS is integrated with ACS, but all of the devices in DCR are not clients of that same ACS server, then you will not see them in any of the LMS applications. They will, however, show up in the Devices not in ACS report under Common Services > Device and Credentials > Reports.

Default credentials doesn't really come into play here. Discovery will add devices to DCR with the read-only SNMP credential it used to discover the device. That may be different that the read-only credential you have configured in default credentials. Campus Manager Data Collection only uses SNMP (read-only) to manage the devices.