Hi Rachel & Eric:
I did this most of this same trip last year and I have some things to offer on the experience.
I left from San Francisco on 9/1, and like you intend, I traveled very light (15 lbs. before water) and ate in restaurants and slept in hotels. I also looked to average a 100 miles a day for the trip.
• Leaving in the beginning of September, going west to east worked out very well for me. No guarantees with the weather but I beat any snow or serious cold through the high mountains and it was never brutally hot. The two disadvantages of west to east in September is that in the early morning you, and the traffic behind you, are going into the morning sun and that the days are getting shorter.
• Counting on averaging a 100 miles a day out west is tough. If you're thinking you'll clear 38 days of vacation for your 3800 mile trip you might want to rethink that. East of Fallon in Nevada for the next 400 miles the towns are 60 or 70 or 80 miles apart. If you're not camping you will have a choice of stopping at the first town in 68 miles or the second town at 142 miles. Southern Utah can also have pretty big spaces between hotels. Once you get to Colorado and beyond it becomes easier logistically to string together a bunch of longer days.
• In a point related to the paragraph above, consider adding a few "off" days. This route will roll you right by the entrance to Zion & Bryce Canyon and I did not stop when I was there. I'm telling myself that some day I will go back and hike them. You may want to consider including day or two to hike the most spectacular scenery in the US while you are already at the gate. Also, there will probably be some day(s) that it's raining all day or the wind is up and against you all day and after 70 miles you'll be thinking to yourself "Enough!".
• Super light/ No camping (aka "Credit Card Touring") has it's advantages. My east/west tour last year was my first touring experience. I just got back yesterday from going from Jasper to Denver and I packed pretty much the same way and on this trip I did average 100/day. But it has it's disadvantages too.... There can be a certain amount of tension with making your hotel by the end of the day. And you may not stop to take a longer look at something cool along the way for the same reason. I intend to continue to travel light but I mention this FYI. Also, I've averaged about $140/day for these trips, food and hotel, though I'm sure it can be done for less.
• I'll speculate from the info you included in your post (not intimidated by long days/ want to travel light) that you two are "roadies" who have decided to do something different. That would describe my background. If that is true, just be aware that this is terrible for your speed. Riding for 7 hours a day at 15 mph makes you real good at riding 7 hours a day at 15 mph and nothing else. My speed was significantly down on all my typical training rides after I got back from my last trip.
That's what I got, I hope it helps.