I usually plan 400-500 miles per day, though I have done over 700 miles in a day but that was in my SUV without a travel trailer.
I use Microsoft Streets and Trips to plan my trips and I've found that if I set the driving speeds settings to slightly above average and schedule a 15 minute stop every 3 hours. This pretty closely parallels my actual driving, so I try to plan a stop +/- an hour of where it suggests stopping for the night.
I usually only make reservations for the major destinations that I plan on and I use MSS&T to plan the dates that I expect to arrive.
Works well for me.