detroit diesel

We are looking at a 2007 Columbia. It has a 14 litre Detroit, 10 speed, 3.07 diff, single axle. At 65 miles per hour RPM should be 1,250 We pull
16,000-18,000 lb RVs and fifty percent of the time we pull 8,000 lb Dodge one ton on very flat terrain. My question is will this engine pull at this low RPM? Also what fuel economy can we expect?