Traveller's RV park just as you arrive in Winnipeg from the east, nothing fancy but easy in and out off the highway, and good for an overnight. It's just off the Trans Canada as you enter onto the Winnipeg By Pass.
When you say headed west you did not stipulate if you are going #1 through to Calgary or #16 up north through Saskatoon then on to Edmonton.
If going #1... Prairie Oasis in Moose Jaw, SK...again, nothing fancy but good for an overnight, easy access off #1....
Also Medicine Hat has a very nice city run campground, called Gas City campground, also easy access off #1.
These all have websites you can look at.
Most of these, in Aug you will probably want to make reservations, asap, if you aren't too late...if you are planning on staying in the Nat'l park in Banff, I would say you are likely out of luck, that would have had to have been booked long ago.
2016 Chevy Silverado 2500 Duramax
2016 Rockwood 8289WS, Diamond Pkg.