The halfway point along the I-5 corridor would be the Red Bluff/Redding area. There are several good, full-service RV parks in those communities, some along the Sacramento River. Check out Trailer Life Directory for specific information on RV parks in those areas.
If you decide to take US101 in one direction on your trip, I would concur with Wayne about the Benbow Valley RV Resort, especially if you play golf. It is in the Humbolt Redwoods, just south of Garberville about 1 mile, and along side the Eel River.
Paul <?)))>< Lake Almanor, CA
2002 Thor Tahoe 23FBGL TT, 28'