Parking RV overnight at trailheads within national parks?

I agree - I would not leave my rig at a trailhead for reasons already identified. I think your best bet is a private CG nearby. They will often have an area for RV "storage" - lower daily rate than occupying a site. If you are towing a trailer, makes it easy - just take the TV to the trailhead (with all gear left at the trailer...). Certainly worth a phonecall to whichever park/forest you are hoping to visit - they will likely have suggestions to offer. ST