I run Vibco Big Bertha DC-3500 vibrators in my dumps. They're not an end all but they sure shake one HELL OF A LOT harder than a Cheetah or Buyers brand I have been around in the past.
Diesel will help usually too but it's frowned upon, that's why you refer to it as EFRA (environmentally friendly release agent)
Asphalt plant I haul out of gets stuff from BG Chemical, called Black Magic, it works good, but is susceptible to freezing, I had thought of getting some of it set up in an automatic truck bed sprayer like they have for a snow haul but the freezing aspect of it cancelled that idea out.
For hauling snow, used glycol works pretty good too, also however frowned upon for environmental reasons.
I have heard good things about dish soap spread in with a mop, but haven't tried first hand.
All of these are helpers, but at the end of the day as a dump truck driver sometimes you've just gotta crawl in there and clean your box out.