A portion of our cross country route went through eastern Oregon, southern Idaho and Wyoming. We were not on an ACA route, but generally followed US Highway 20. Our start date was later than your anticipated start. We started the last week of June. We used lightweight 25F bags that were great early in the trip, but were overkill later on. We also used Therm-a-Rest pads which work well all year round. Insulation under you is as or maybe even more important than that on top of you. Our problem was not with cold weather. It was the heat that was challenging to deal with. We hit some really hot temperatures in all all of those states. Daytime temp reached the 109F several times with temps running in the high 90's,s to low 100's on many days.
I've worked and played quite a bit in the Oregon High Desert, and while the evenings are cool, they are not extreme. the other suggestions of a warm layer will probably see you through. We usually carry a lightweight down or synthetic fill vest that supplements the sleeping bag, is nice to wear in the evenings, and can be worn for the first few hours in the morning. It compresses to a smaller size than a fleece vest, is ligher and warmer. As you get further east the heavier clothes can be mailed home when no longer needed. Most of it can fit in a $15 Postal Service flat rate box.
There is not much shade in some sections--between Bend and Burns, OR.
I used the Timberline 4 tent for a number of years ( actually wore it out
) . While you are carrying some extra weight with it, it is a great tent. Using some of the techniques the other posters suggested will likely get you through the coldest part of your trip.
The same tent and pads (Therm-a-Rest Prolite 4) we use for bike touring, backpacking, spring and early winter ski trips. The sleeping bags change from -20 to +45F, depending on the season. Different tent for mountaineering and winter camping.
