I thought if ClientAliveInterval was set to the default of zero then these messages are not sent and no timeout and disconnect of idle ssh sessions is configured. So I'm not sure this is the issue in this case.
Did the changes suggested solve your issue?
Is the ssh client connecting directly or are there any firewalls in-between?
I know you mention this is affecting ssh clients, but do console sessions also get logged out after an hour?
Have you checked if there is any timeout set on your ssh client?
(There is a ServerAliveInterval for ssh_config, which you might want to review)
Is this the only RHEL host with this issue?
Have you tested from a different ssh client?
For ensuring an ssh connection remains in place you could look at something like autossh:
Hope this helps.