One or two of our solar panel mounting knobs were loose last November when we put our Ollie away for the winter... just 3 nights camping from factory pickup. Checked them all and put it away with sufficiently tight nobs for the winter. We've maybe put a couple of thousand miles on it so far this season, and there were 2 knobs loose again. Not sure if it was the same 2 knobs as before or not. All knobs had a lock washer installed, but they were compressed between the metal bracket and the plastic knob so not sure if they were able to fully achieve the intended effect.
I added a stainless flat washer to each knob, and implemented John Davies' locking solution described above (thanks, John!). Easy, cheap and effective.
On a side note pertaining to tilting the solar panels to facilitate water runoff, that does not appear to be possible on our 2018 Elite I as the box holding the connectors is located right underneath the panel and is so tall that there is insufficient clearance to drop down either side of the panel a notch in the bracket. However, when the trailer is level (as determined by the bubble on the tongue jack/stabilizer) there is a slight forward tilt to the solar panels due to a slight slope in the top ridge of the roofline. When level and at rest, water flows forward and off of the panel's top surface.