Thanks so much!! The isnull works!
I also did not realize the iif statement evaluated both true/false parts. So now when I evaluate the is null I populate a string "SetFromSomeField" and then use the Replace() to set it, works with Year() too.
Run_order | Value_info
1] | [UNITDESC] & ", MH DEPTH " & IIF(isNull([MHDPTH]),"UNKNOWN","SetMhDpth")
2] | REPLACE([UNITDESC],"SetMhDpth",[MHDPTH] & "FT")