Wow, that is the toughest conversion I've ever done, litres per 100 km to mpg. Actually, I found a good website and it was easy, but not before it hurt my head a little.
I have a 1996 Coachmen with a similar top, believe it is a 350 cid V-8, although still need to confirm the displacement 100%. Tend to get between 12.5 mpg and 13.5 mpg, 14 is my goal but keep falling short each tankful for one reason or another. Drove with a fan cover last trip because a lot of rain and wondered if that hurt, but I am determined to get everything right to get up to 14. Had a big tune up over winter. Guilty, lead foot, I will go 65 to 70 mph if the speed limit allows and everyone else is. Also, I hear a hood bug shield may help. But right now, I can honestly only report about 13 mpg.