Although the route Marv described is doable, I wouldn't do it for the place you want to end up. 49 between 120 and 140 is steep and windy, but doable. It's also taking you back uphill and East into Mariposa before you pick up 140 so you can go west.
If you want the most direct route and easiest drive, I'd come down Highway 120 as before into Oakdale. Take a left on South Yosemite Ave in Oakdale. Go South on that road and it's going to change names a few times. You can chose to drive through Turlock, or I'd probably take Keyes Rd over to 99 and then go South on 99 to the Lander Ave/165 Exit. Go South on 165 into Los Banos and take a right on Highway 152. That road will eventually turn into a freeway. Take it to 156 and then 156 to 101 South. Exit in Prunedale on 156 again and then follow it to Highway 1 and then to Marina Dunes.
I don't want to give you too complicated of directions, but if you like freeways over two-lane country roads and cows, once you get on 99 off of Keyes Road you can take the West Main exit and go west on West Main. Take a left on Crows Landing road and then that takes you over to I-5. You can then take I-5 to 152. This route is probably a few miles longer, but it is faster as you're on the freeway more.
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