It depends to a great degree on where you are and where you are going. Interstates or major highways are a lot different than smaller roads of course. And, it depends on the part of the country you are driving through. For instance, we often drive 500 miles a day heading out west from Oklahoma City through Texas and New Mexico, wide open spaces, straight and no big grades. Otherwise, 300-350 is just about it.
Looking at OP KenMast's rig and toad I think he is right on the button with his calculations. We bought these things to enjoy, not set land speed records.