My trip is not until July, but I have reservations at a nice city park in Claresholm, Alberta. Maybe you wanted to get further along the way, but this one fits our plans just fine.
There is an RV park at Lethbridge, and at Champion (Virginia Mitchell Memorial Park)
We will be crossing at Sweet Grass and spending our first night in Canada at Clareshom.
Claresholm parkSend me a PM if I can help. I have good route information for late Summer and can look up campgrounds if that will help.