Very interested in the answer also. I would point out that the two obvious rv park choices nearby--although fully priced--are extremely well-located, and the prices are not all that bad. Very common to find higher prices and worse locations when dealing with mountain resorts. For example, the only RV park near Mammoth Mountain is $1500 for a month in January.
That said, I would advise checking each individual ski resort's web site for their overnight RV parking policies (and report back). All of them have huge parking lots and many of them have policies INVITING guests to overnight there, usually for a small fee.