Yeah, it's sad. Those people who set up shop and camp at overnight parking spots are the worst! They ruin it for everyone.
The solution was to park in long term Via Rail parking just outside the city on the South Shore. A short train ride right into the heart of things is right outside the RV door.
$15/day, plus train tickets to/from and Metro cards.
So, it all worked out.
This is the exact reason I went with a truck camper over anything larger. I can squeeze it into normal car parking. It's a godsend for visiting places like Montreal. :)