Yosemite in an RV without reservations around June 25

We have use of a friend's RV (33') for 9 days from June 23 - July 2, and want to go to Yosemite. Alas, the sites all appear to be full (in Yosemite and nearly everywhere else nearby), but we got reservations at Bass Lake/Lupine campgrounds for 4 nights. We're pretty flexible, and my kids are dying to go to Yosemite (alright, I admit it, so am I). My thought was to try and take a day trip up to Yosemite and get into a campsite that's first come, first served in Yosemite, one which also allows RVs.

I'm happy to eat a night or two at Lupine if I can use it as a staging ground to get into Yosemite, and then stay there the rest of the time, but I just don't know how hard it is to get one of those campgrounds. I believe that only Bridelvail and Tuolomne Meadows have first come, first served for RVs of 33', and I'm not sure when Bridalvale will even be open (I thought the site was avaialble for a horse reservation for that time, but it's impossible to see if the whole campground is open). The NPS website said that it's tentatively set to open July 5, but does that mean it could be earlier OR later? Is it worth trying to go there?

Is Tuolomne too far to try to get to and risk failing? I figure since Bass Lake is only about an hour or so from the park, it's not too hard to take the drive to Bridalvale Creek and not get a place, but Tuolomne is a lot further away, and that would be a major drag.

A couple more questions:
1) Are there cancellations at the reservation-only sites that you can get in to at the last minute?
2) If we don't have a separate car we're hitching along, will we lose our campsite if we take the RV out to dump it and fill up on water at some point? 3) Is there some way to keep your campsite when you leave it for a little while (I understand it's different when you leave your tent there all set up, but here our "tent" would be riding along with us).

I'm curous about the thoughts of some more experienced people.

Thanks.

If you get to a campground in the morning when people are leaving, you should be able to get a site even at Tuolumne.

Thank you to those who posted. FWIW, we drove in through Wawona on Thursday morning, and the person working there said that there spots available at both Wawona and Crane Flat. Not knowing which to do, but figuring Crane Flat was at least closer to the valley (and would take us to the valley for a brief moment), we had them give our names to the Crane Flat folk, and we drove up there and got a spot. There were also spots available at Tuolomne Meadows for the whole time, had we wanted to head all the way out there.

After 2 days in Crane Flat (Sat morn), we really wanted to check out the valley, so we drove there, parked the RV in day use parking, and rode our bikes around the valley, going to the lower falls, and climbing the rocks with the kids up to the spray at the bottom of the falls. At 2PM we strolled by the Reservations desk, and put our name on the list. We were #23 on the list for places, but by the time it came for them to actually call the names at 3PM, many of the people never returned (many had put their names on there at 8AM). We were 7th in line to get a spot, and got ourselves a great RV spot in North Pines, right on the river, for 2 nights.

We were thinking of spending another night tonight, but were tired after a week, and had things to do at home, so we actually got a spot for tonight, but after thinking about it, returned it an hour later or so for a refund. Part of the consideration was that the new spot was a much more difficult to spot to get into with the RV, and partially because we were on the fence about staying in the first place.

Thanks so much to all who gave advice, the trip was awesome (but I'm preaching to the choir, aren't I?