Leave car near marina
IIRC, they separated day use from longer-term parking. No charge.
When I did the Flaming Gorge trip, I arranged for parking (truck and trailer) in a concessioner lot surrounded by chainlink fence and locked at night. They charged a reasonable fee. If you’re really worried, you might ask the marinas if they have something similar at Powell. It’s probably not a big risk off-season, though. Might depend which location you start from (i.e., how close to towns and schools).
Disagree
Lake levels are NOT the lowest ever. They are at least 50 feet higher than about 2 years ago when I did a 6 day trip. We launched about 30 feet below a boat ramp and that boat ramp was being used when I was there last October.
The “torpedo tubes” are easy to make and I even ordered the RV parts online. These and the wag bags work well.
I second using the long term parking at the marinas. the marinas see some traffic at that time of year and there will be other vehicles parked there as well.
Camping does change with the lake level, but you will find it along the shores. Get a good topo map of the lake, about $10 and worth it.