I am trying to join my NAS to Active Directory and I am getting the following error…
[EFAULT] The DNS query name does not exist: X.X.X.X.in-addr.arpa.
This sounds like you don’t have reverse zone entries for your DCs.
Improbable with Windows DCs. @Jeff_Curtis does your TrueNAS use the AD DC(s) and only them as a DNS server?
We had someone report in that this same text a few weeks ago due to this (reverse lookups of their DCs failed). I was honestly surprised that the domain was in any way functional, but apparently people can limp along pretty well with broken DNS somehow 
No i´m not suprised. Because i had the same thing, i reinstalled a truenas host with a newer version and the same thing happened to me. The old installation was joined to the AD, but when i try to join the new installation to the AD, it´s giving me the the reverse dns error.
So it´s not about the setup at home beeing weird, it´s about Truenas made their code non-functional.