Park at Fairview Mall on Sheppard Ave west of Woodbine and take the subway. Or check if there is parking available at the York Mills subway station, south east corner of York Mills and Yonge Street, and then take the subway. The common denominator is the subway - it would be zany to drive an RV in downtown Toronto. Be sure to check if you might save a couple of bucks by buying a day pass or a family pass on the subway.
Just occurred to me: there's a municipal campground in the east end on Kingston Road called 'Glen Rouge' which might be convenient. From there you could drive a bit east to the Pickering GO station and park at the mall, or west to either the Rouge station or Guildwood or a bit farther to Scarborough, depending on which seems to have parking available - that's a commuter train line and the lots fill quickly and early. If you've driven that far west you're just as far ahead to park at the Warden subway station which always seems to have lots of parking for only $4 or so.
Forget about that campground, might be a KOA, which bills itself as 'Toronto west' - you might as well commute from Florida.