I agree with the good Doctor. If the slides do not have stabilizer jacks (and most do not) don't put anything under the slides. If for some reason the trailer sits for extended periods, the tires do deflate and will cause the jacks to put upward pressure on the slide.
It may be miniscule but could be enough for water to get in.
My opinion and worth what you paid for it.
2015 White Hawk 20MRB
2011 F150 Non EcoBoost (5.0)