Kenneth, there is no easy answer to the problem.
Since the use of asbestos was made illegal in the brake pad compound the replacement compound is much harder and "dusts" readily.
Normally it is an accumulation of dust which causes the squealing, but also rust build up as well, and with vans sitting idle for long periods of time, the drums start to rust, which is rubbed off when you next tow.
Roughing the pad surface up will do nothing to stop it, only shorten the life of the brakes.
Unless you clean the brakes with copious quantities of brake cleaner, and the drums every time before you tow, there is no way to totally avoid squealing brakes.