Last year (winter) I went on a 6000 mile trip towing my 28.5ft tag loaded at around 9000lb. I drove the northern route to Phoenix, flagstaff and down,then the southern route back to IL
Going out I averaged 70mph coming back was 60mph. This was hand figured at every stop as well as at the end.
Without towing and traveling I run 72 and will get 17.5-18 around town and normal driving I get 18-18.5