When I added my follow-up I didn't realize that I had at first asked about an RV that I did not buy. However I did, soon after, buy an older RV (2006) that the question still applied to. So, yes I put in synthetic oil when I got it and it never had any leaks. There was no loss of oil when I just recently changed the oil again.
I learned that I had to drive under 60mph to get 8 mpg; over that got me 6.4 mpg. So, since I am retired and was on vacation, I just took my time & we thoroughly enjoyed the trip.
Thanks for all the answers. Al