I got this solved.
Apparently, the xml file doesn't get updated with the postgresql settings if the database can't be reached. So this was a symptom, not the cause.
The cause turned out to be a simple firewall rule that was incorrect. In testing I had added rules to allow all traffic, but that wasn't sufficient. Once I completely disabled the firewall, nuxeo was able to connect to the database and everything worked great. Then we were able to recognize the bad rule.
