I had previously checked the HWH springs and the rear mud flap hitting anything. Nada.
Crawled around underneath again this time with DW sitting in the seat and step cover closed. Until your suggestions I hadn't considered the front metal debris/splash guard
. Plucking it towards the rear with a couple of fingers make about the right noise but no thump. Then I realized the Kwikee step was extended (step off). Had DW retract it and did the test again. Sure enough, thump then twang. The guard was hitting part of the retracted step frame. DW confirmed the sound/feeling from the co-pilot perspective.
Given how rigid the metal is the event trigger is probably a harmonic being setup from the chassis and possibly the addition of just the right air pressure from forward motion and cross-winds.
Looking more closely I found only about 1/4" clearance between the guard and the step. I pulled back on the guard gingerly to bend it towards the rear a little and managed to increase the clearance to about 1/2". As added insurance I used Locktite liquid metal to glue a small piece of foam pipe insulation to the side of the Kwikee frame.
Manual testing says the problem is solved. We'll find out on the next trip starting 8/10 if works under driving conditions.
Thanks again for your suggestions to point me in the right direction.