seattle to flagstaff next week

hello to all, here we are on the eve of our first big trip with our new to us combo, 1999 4 door FL-60 freightliner tow rig and 07 forest river 38 foot toy hauler, let me back up a little and say we have survived the first month of full timing together after some down sizing on both of our parts, there have been some snags, our water hose froze the first cold night, now it is heat traced and wrapped in foam, we ran out of propane, what a cold night that was, now we always keep the second tank closed so we at least know that 1 tank is empty.
Now back to the title of this post, weather permitting we will be heading out Tuesday over the I-90 pass and pick up the I-84 at Ellensburg, that will take us to Salt lake city, DW will have to drive the car to the border of Idaho where we can triple tow from there to Flagstaff, south of SLC we will jump on the I-15 to Vegas and spend a couple of days there in an RV park after all the boondocking ( we are planing to stay at walmarts on the way at night ) out tanks will need to be dumped.
Some other places we want to stop at are the Route 66 Museum in Kingman, we plan to only drive 350 miles a day or 5 to 6 hours of driving, with rest stop that 5 or 6 hours could be more like 8 hours, other medium duty truck owners that have Freightliners and close to my combo claim 9 to 10 MPG, I will post up mine at a later date, thanks for reading everyone Lyman & Kelly