Best time is relative to what you want to experience. For me, it is shoulder season to avoid crowds.... Late Aug into Sep-Oct.
BTW CG options for the largest rigs in the Yosemite general area are sparse to say the least.
Scenic Route above has the Yosemite recommendations (esp Kings Canyon
Lee Vining Mono Vista is about only choice that side of Yosemite. Tight for 45ft and towing - only a few sites there that can handle (watch the trees there they can be low if you are tall). It is a great location to stage the eastern side of park and Mono Lake from.
If you do Tioga Pass (especially eastbound) with RV make sure you are extremely familiar with you rig's mountain capabilities. I am a habitual mega mountain runner and that is still the only pass that I have to manually force first gear engine braking and still nearly smoke the service brakes. At 5-10mph or so it'll take some time. I'm a 45er and towing. Yosemite NP roads do have some length/height restrictions on the west side of the park.
On other side (western) of park is the Yosemite Lakes CG in Groveland - they do rent to non Thousand Trails members if space available and that lets you do the other (west) side of the park more conveniently. Big rig friendly...
Add Mammoth Lakes as a stopover if you hike/bicycle/fish. Mammoth Lakes RV CG.
Run through Death Valley as well (esp if in Sep/Oct). It'll still be hot, but no crowds and an interesting experience. Play a round of golf there. Most likely will not need any reservations. Furnace Creek CG.