First thing I would mention is the travel times. Niagara Falls to Lake Placid 334 miles suggests you are probably taking rt 31, or 104. to Watertown and then 3 east to Saranac Lake Nice routes. but you are not going to maintain a 50 mph average. Better add on another hour or two. If you are having to choose time over sight-seeing you would be better off taking the NYS Thruway to I-81 north to Rt 3. Probably 20 miles longer but a lot less slow traffic and delays. BTW, top off your fuel in Watertown, Fuel prices go up 15-20 cents a gallon in the mountains.
Camping in Lake Placid, The one really good campground that was close to town was bulldozed some years ago for expensive condos. The only other close in choice is Cascade. It is largely a permanent mobile home park. They do have transient sites . It is surrounded on 3 sides by an airport, a landfill transfer station and a sewage treatment plant. I have stayed there, it is ok for a weekend but I wouldn't want to make a summer of it. There is a KOA about 10 miles out of town, I have driven by it but never been in it.
From Lake Placid to Bar Harbor. Again your travel time may need to bump up another hour or so. I am assuming you are taking the ferry across Lake Champlain and then US 2 east? You will be passing Burlington Vermont and it is worthy of at least a 2 hour visit downtown. some years ago they closed several blocks of downtown turning it into a pedestrian mall. Lots of unique shops and great food choices. The band Phish is from Burlington so you know the town has to be cool
If you can get an extra day, stay and see more of Burlington and also visit the Shelburne Museum, you will likely drive past it from the ferry
Heading east from Burlington you will pass through Waterbury VT, home of Ben and Jerry's Ice cream, you gotta stop there! You will also pass close to Mt Washington in NH, another worthy stop on your way to Bar Harbor.
Bar Harbor campgrounds there are several good ones. I like the KOA Oceanside on the north end of the island. We will be there for 5 days beginning August 1. Just up the road a mile or so is Narrows and Narrows too. Those are nice campgrounds also.
Heading to NYC I would suggest 95 to 495 to 290 to 90 to 84 to 684 to 87. Go west over the Tappan zee( Cuomo now) bridge to Garden State Parkway south. Your travel trailer is legal on the Garden state as long as total rig length is under 62' You also want to be under 13'5" height (lowest bridge) Again this is longer but a much less stressful route.
The KOA Madison near Pittsburgh, we stayed there last October. It is kind of run down but there is few choices around Pittsburgh. We might stay there again in August. There is a fairly new one north of Pittsburgh called Mountain Top that has less amenities but looks a lot nicer and is 10 miles closer to the area of Pittsburgh we will be visiting. I'm going to read up on that and see it works for us