I recently converted my dinette so I can swivel it when on the road so it is easier to get by without putting out the slides. It was really easy.
The top is held on to the base with 6 screws. I purchased a 6 3/4 inch bar stool swivel from Lee Valley Tools here in Canada (see site below) it cost $12 and is very heavy duty. They are made in the USA so should be available at some hardware stores down there. I bolted it on to the top of the table base with 4 x 2 inch 90 degree brackets, then turned the whole table upside down to screw the swivel onto the underside of the top.
I then screwed the table back down to the floor
As the table slides back and forth it is really easy to slide it to one end then swivel it and it misses the window and gives you about 12 inches clearance from the counter top.
I have also fashioned a lock to hold it in place.
Click on the links below for photos
Below is the link for Lee Valley which shows you what kind of swivel I used.