This is really messing with your reservations but what if you backtracked from SD to Moab and then come across Tioga Pass into Yosemite from the East. You might be able to get more time by switching your dates and hopefully being in Yosemite midweek.
Also, just checked again and your RV is 38'. That will limit you in Yosemite. I didn't think they could take over 35'. I just tried to check and I couldn't find a master list giving length restrictions. I found Wawona's limit by clicking on an available site.
BTW, fireworks are great in SD for 4th.