Yosemite or Gand canyon and Lake Powell?

I would always recommend Yosemite over any place that I have been. However in your case it will probably be just about impossible for you to get reservations in the valley this year, so I would suggest going to the Grand Canyon. Take this year to learn how to economize your use of your RV expendables, water, battery, etc. It might take a bit of doing, but 5 days dry camping in a fifth wheel isn't an impossible task. Start by turning lights off and using the camps restrooms as much as possible. Maybe consider installing LED lighting in the 5th wheel. Use a water thief to fill your drinking water tank. Get a couple of LED lateens to use inside the 5th wheel to further save on the battery. If you don't already have a generator, you might want to consider solar panels. A 200 watt system will most likely allow you to keep two group 27 100 amp hour batteries charged. I camped with a 5th wheel plus slide out for over 12 years using only solar. It takes a bit of an effort, but it is by no means impossible.

You really want to stay in the valley if you can. The kids will have a great time. One thing others have not mentioned, is that the valley campground has plenty of places to eat and a grocery/general store if you are worried about being able to carry all the food you might need.

The NPS has just come to agreement on what they are going to do in the valley and they will be keeping river rafting on the Merced and actually adding camping spots in the valley camp grounds. What ever you do, you should love Yosemite.