My 1988 560SL gets about 17mpg 90% highway with the hard top, 15 with the top down. If I drive highway only hard top installed, like a 200 mile trip it gets 19. I would expect a 560 SEL to get less than this. Another option might be a 420 SEL which was available in the 560 vintage or a 560SE, a little less room in the back.

Sorry to hear about your loss and good to see you are OK.

John Roncallo