Just 10 days ago I booked a site for march. I checked 2 days ago and there were sites for the 23rd for a few days. I will tell you we have camped in the valley of Yosemite at least a dozen times in the last 20 years.
December you can get in no problem and the crowds are light. Watch the weather closely. March can be pretty easy to get too.
There are only so many sites for the bigger RV.s If you are 35 and below it helps as there are many more sites for that size.
As far as camping outside the park it is a 30 minute drive into the valley. There is no better way than to stay in Yosemite valley. Think having coffee outside looking at Half Dome. It does not get any better. Due to business constraints we can no longer visit in the summer. But we have many times. You can ride your bike or walk anywhere in the valley floor and are close to all the hikes.