When analyse RFC connection issue, the first thing we need to be clear is, what is RFC client, what is RFC server, what is communication type between RFC client and RFC server.
For SAPXPG_DBDEST_<SAPDBHOST> RFC:
Basically, The root cause could be at RFC client - AP server gateway side, RFC server - SAPXPG on target DB server, communication type and network issue.
So, for SAPXPG_DBDEST_<SAPDBHOST> RFC is not working issue, normally, depending on configuration, you could troubleshooting it as below.
if the communication type between RFC client and RFC server is OS rsh or ssh(default setting):
if the communication type between RFC client and RFC server is "Gateway connect to remote Gateway"(a standalone gateway is installed on target DB host and you configure in "Gateway Options" of the RFC destination):
Related SAP notes: