I believe the DNR thinks you need one for a dual registered rig. Hence the language in thier FAQ on the Discover Pass website. The OHV tag exempts you from needing one on the trail, but if you parked the rig in a parking lot or motored around recreation facility roads, then you need a Discover Pass (technically).
I've heard guys have tried removing the street plate on the tow rig after they park and only display the OHV tag. Not sure how that has worked out. I have a Discover Pass for my tow rig, but only OHV tags for my 4x4 rig. I just unload my 4x4 & hit the trails, but I don't drive my rig on the paved roads...just dirt.
I think the new version of the Discover Pass will have a 15 minute grace period for vehicles entering a park...after 15 minutes a Discover Pass will be required. Maybe this will help clear up the problem...enter the park, hit the trails with your OHV tag & then leave immediately. No Discover Pass required for a dual registered rig.
WOHVA | PNW4WDA