I do have a few ideas. Do you want to fix it up to sell or to rent out or for you to live in?
1978 is along time ago. I would look at the functioning of the appliances, HVAC, and how outdated the stove, range, and facilities are.
I would look at the frame and the actual structure and the conditions of the hitch, wheels, and possibilities of it being moved with normal ease.
Surely they have some sort of blue book for trailer homes like the have with cars. What you need to find out in your market is what a 78 sells for and in what conditions. You have poor, good, and excellent conditions. Find out what each category is selling for.
Without seeing it, and if it has not been updated recently, then you need quickly appliances, stove, range, refrig, micro wave, toilets, and maybe HVAC units. This is alot of money right up front.
You can do wonders even if not a DIY with new paint, new wall papering, new flooring, and general sprucing up. This you can do your self or hire someone to do it and you chip in with your labor.
I will not attempt to give you any dollar figures because I want to see it first. This could be a thing of potential or a complete "dog". You need to do some research and leg work first.