Did two cross country trips to Yosemite in 2010 as my son was working as a chef at the Ahwanhee Hotel for 5 months. He will be back at the Ahwanhee this May in case you need any local help or advice.
There are only 12 campsites in the valley for a 40 foot MH. If you look at the campground reservation site, all three campground have projects going on. North Pines (where we stayed) is closed indefinitely while they work on an upgrade to the utility system. Upper and Lower Pines are closed for a period of time in May while they reseal the roadways. All sites have also been reserved already as the window for May 2011 opened up 2-3 months ago.
Since you are going in May, Tioga Road will still be closed so camping in Lee Vining is not an option. I have driven the Rt. 41 route out of Fresno a few times and actually prefer that route. Remember to fill up just north of Fresno as there is NO gas/diesel in Yosemite Valley. We stayed one night at the Coarse Gold KOA but that may be a little far to do day trips into the valley. There is a campground in Oakhurst which is closer but I have not stayed there.
Rt.120 has a wicked climb by Groveland. We did it uphill and I would not want to do it downhill! Rt. 140 has a length restriction but you can go through with a 40 footer (you will need to unhook your toad if you have one). There is a campground in El Portel but, based on the camground listing, not sure if they can handle a 40 footer.
If you stay outside Yosemite, you will definitely need a toad for your daytrips as parking is limited for big rigs. But last May, I remember my son calling me to tell me "Yosemite Falls is cookin' " so don't miss out on a chance to see it!
Tom and Katharine
'07 Winnebago Tour 40TD, 400hp Cummins
RVing for 19 years & 150,000+ miles