Some vans of this vintage had couplings designed for an electric reverse catch. This was a solenoid operated plunger which push a rod vertically down through a hole in the hitch casting and prevented the damper spring from compressing. The solenoid was 12v. and operated by either a switch on the dashboard, or by connection to the reversing lights.
I think you hitch should have either the swing over collar (more likely) or the fitting for the electric version. If the latter you may be able to use a round screwdriver or similar pushed down through the hole.
If there is neither, then you ould make up a sleeve to fit over the coupling, keeping it in place while reversing with a bungee or similar. Remember the hitch must be fully extened first and then kept in that position. Don't forget to remove the device ( or swig the collar out of the way afterwards).