Mill Creek can be hit or miss on availability, at times many open sites and times when it is full. More towards the middle of the week is ideal. Lundy Canyon tends to fill up on the weekends and empties out on the weekdays. If you PM me with an Email address I will send you some pictures of the area. I don’t have any of Mill Creek as we have always stayed in the resort, but many of Lundy Canyon.
If you can’t find a spot at Mill Creek there is a nice RV park located in Lee Vining called Mono Vista. It’s a nice park, but right in town and no privacy between sites. We stayed there once until a spot opened in Lundy.
Let me know if I can help in anyway. We have taken friends to Lundy and they all have falling in love with the area, I’ll bet you will also.
PS. If you are into fishing this is one of the best places in California for trout. Minutes from your door.