Drain valves are in my heated basement, easy to open and are about 1.5 feet from the outlet.
KOA site for Revelstoke shows May 1 to Oct 1. Still worth a call.
2x the comment to use generous amounts of blue windshield washer / de-icer, if you go that route. It doesn't protect well if diluted too much.
We have lots of -20F washer fluid (~ 37% % methanol) in the stores around here. As can be seen below once diluted 1:1 it only protects to 10F; a 1:2 dilution only protects to ~20F
Once in BC or Alberta, we usually stop by Canadian Tire and pickup a gallon or two of the -45C (-50F) washer fluid (~ 50% methanol) for the truck wiper system.
If you use that in your toilet / black tank, it gives you a little more room prior to freezing as seen below.