I live almost next door to Priest Lake let me know your plans and I will try and help you. Most of the camp grounds are booked in the summer there some open boat in sites in the upper lake. I don't know where I would leave a trailer but will find out. I also PM you.
There's parking at the launch ramp need to pay at the camp ground office. Don't miss a look at Upper Priest Lake you get to it at the north end of the lake. Take the thorofare to get there. You can read about it on Bill and El's website here http://www.cruisingamerica-halcyondays.com/priest.htm