Sorry for the delayed response; had some other things in Method to work on.
I figured out how to do this and figure I'd leave some instructions for people to reference:
1. Add a loop through table action that references the Activity table
2. "Where" conditions for the loop set to grab whatever ActivityType you want the last date for . E.g. if you want the date/time of the last email sent to a customer set ActivityType to "Email Sent"
3. Add "Retrive record from table" action that grabs the Activity Start Date (or whatever date you want) from the Activity table BASED ON Activity.RecordID = [name of the Action Result from step 2]
4. Insert it into a textbox
This works because it will grab all the Activity RecordIDs that have ActivityType = Email Sent but it will be ordered by most recent to oldest. So, the Retrieve record from table will use the most recent RecordID FIRST; since textboxes only take 1 record, the most recent RecordID will be inserted into the textbox and you can use that recordID from there.
Thanks again for the help.