Pulling a 5th wheel from Ottawa to Fort Myers

First advice would be to watch the weather forecast for your route down. If your not on a fixed schedule, try to give yourself about a 3 day window (depending on your driving habits, time/miles) to get south of the mountains of Virginia.
Can't recommend a route for your as we take I75 south from Detroit/Windsor. We generally try to leave early in morning to get to KY for the first night. This is about 10hrs driving for us, but it is generally the first open campground in winter.
Dewinterizing is normally done in TN or GA, depending on temperatures and forecasts. We use bottled water in the RV and pink for flushing.

Beverley and Ken

I always use 81s to 66 to 17 drops you off on 95 in Fredericksburg va I am from pa

We just made our second trip from central NY (near Syracuse)to Ft. Myers Beach. I recommend staying on I-81 as long as possible, thus avoiding I-95 and the traffic congestion surrounding Washington DC, Baltimore MD and Richmond VA. Stay on I-81 past Roanoke VA, then take I-77 to Columbia SC, then I-26 towards Charleston SC until it intersects I-95. From just across the FL/GA border, I take the exit for Yule and SR A1A which leads to US 301. US 301 to Gainsville where I get on I-75 for the last of the journey to Ft. Myers. If you don't want to deal with the SR-301 two lane roads, you could stay on I-95 to I-4 to Tampa to I-75, but the SR-301 is about 100 miles shorter.

You'll have a hard time finding open campgrounds until you reach southern PA. South of there should not be a problem. On my first trip, I made reservations for each night at campgrounds along my route. Found that weather, traffic, potty stops, etc can really affect how far you travel each day. Now we just go until about 1 hour we want to stop and use the RV GPS to find campgrounds 60-100 miles ahead on our route.