Not sure about 1986. Our hitch is set at 19" for our 1973.
The ball size should be 2-5/16".
Other questions would be:
What is the age/condition of the trailer tires?
Do you have a brake controller installed in your tow vehicle?
Have you verified the towing capacity of the van? IIRC a 34' Avion is going to be in the 10,000 lb. +/- weight range.
Here's a link to the manual for the 87 year model which is similar to yours may help you.