I would think 6 is a bit low. I would have expected maybe 8 or 9 from that combo.
Your RPM doesn't seem too out of line. I'll assume you were on flat ground since you are in IL. Depending on gearing, speed, etc. you should be able to cruise on level ground at a lower RPM I would think.
I used to drive a 2015 Chevy 3500 with th3 6.0 that would get about 8 MPG towing a 24' flatbed trailer with various equipment and supplies on it.
My 2009 Chevy Suburban with 6.0 gets about 10 MPG towing my 7,000 pound boat.
