How much air did they put in the tires? How much does the placard besides the seat recommend? If they put the max sidewall pressure in them (30 more than recommended) you'll be lucky to make 300 miles/day. I would weigh at a truck stop and adjust pressures per the axle weights. New tires and too much air makes for a poor handling rig.
I really am trying to answering your question as it depends on how well your rig handles and how well you handle it. If you are not used to driving something with a long wheel base it's easy to over steer it. I still do the 1500 miles to CO in two days BUT I have a DP that handles very well with very little steering input. Your mileage will vary. GOOD LUCK