Sounds like a good plan - who are you renting from and have you verified you can rent from one location and return to another? I don't think you can do that with El Monte RV, but maybe Cruise America will allow it.
You will need to leave early as possible to make it from big sur to Hearst Castle. Check the tour times to make sure you will be able to get there. Plan on a couple of hours for Hearst Castle at the least. You will be spent by that time, and I would really think about staying a couple miles south at San Simeon State Park - your choice, but that will be a really long day. Morro Dunes is about 40 minutes south of Hearst Castle.
I highly recommend stopping at Julia Pfeiffer State Beach - Waterfall. Check it out here - http://www.centralcoast-tourism.com/julia-pfeiffer-burns-state-park.htm
If you can't find parking on the road before the waterfall (heading south), there is a decently large place just past it around the corner that should be relatively open on most days. The waterfall is where the McWay Creek crosses the 1.
The only real place to get fuel at a high cost between Big Sur and San Simeon town is a place called Rugged Point Inn and Resort. The pumps aren't the easiest to get into, but not the worst either.
You will be passing a few light houses too. Plan on stopping a few times to take pictures - the best pictures are around every single corner on that road.
I drove my truck down that road one year and stopped too many times to count. Not as easy to stop in a camper or with a trailer. Good luck and keep us updated once you get back.
