Thanks so much everyone, some great advice! Sorry forgot to say we are renting a C class motorhome with a generator, probably from RoadBear. Looked at Cruise America but they seem to be pretty hit or miss in terms of quality... anyone had any good or bad experiences?
The new plan; a plan A, a plan B and a plan C!
Plan A (good weather) - stay in Yosemite valley
Plan B (cold weather) - stay in a park outside the valley
Plan C (terrible weather) - go to Napa valley and drink wine instead!
All about flexibility and being prepared I have learned from responses. Have noted lots of the advice re following advice before we travel, sleeping bags, wool hats and socks, house slippers and blankets to close off the cab. Thanks again everyone, this is a great forum! (Might be posting more questions as we get closer)