[sourdough]

As has been said a vented, stackable combo is the only way to go...but that doesn't address your issue.

The unit being in a slide greatly increases the problem simply because the slide itself is "suspended". Balancing, load etc. have been mentioned and help. I use a tripod stabilizer at the pin which helps greatly but my w/d is in the nose. In your case the wiggle will come from the movement of the slide and the way it affects the rest of the trailer. Xchocks help (tires will jiggle), tripod stabilizer, JT strong arm stabilizers (BAL makes the same thing)...many options. All depends on how badly you want the "shake" gone - and you will never make it "go away".