Yep, Pam has a strong point.
In answer to your question, I will ask a question and then give you some answers.
Where are your research skills?
1980, you have HUD data plates, you should have an interior data plate as well. This information can be traced back to the MFG who can give you the data you are asking for.
I hope you are not charging a lite fee, my personal property appraisal reports on MFG start at 1000 bever pelts.
Unless you have done a number of these both as fee simple, lease hold or personal property I would advise you working with someone who has. You can get your tail feathers really burnt off doing these kinds of PP MFG subjects.
You should also visit a number of dealers and check thier inventory on their back lots and the asking price for those units as well.
You need to ask a lot of question from the get go:
Is this unit staying in the park? If not where is it going to be moved to? What are the cost? What is lot rent? What is the condition of subject to the sales you have selected as comparables? When was anything updated? The list can almost be endless.
Hope this helps!!!!!!!!!!!!!!