Plan to visit Universal in December for the 1st time and would like to know if I can park at one of the on site resorts and use their transportation to the parks
Yes you can but it will cost you $24/day at Portofino, Hard Rock, Royal Pacific and Sapphire Falls. It would be $20/day at Cabana Bay. These are the guest parking fees.
And the main parking garage lets you park under shade. And there are moving sidewalks on the way to CityWalk. It will save you time, unless you have other reasons to be parked at the resort.