Seneca Niagara Casino in downtown Niagara Falls, NY, has a large parking lot on the east side. They might allow overnighting--call them to make sure. It looked like the northern part of that area was used by tour buses, and the southern part was used more by cars.
I didn't see much in the Buffalo area for your usual free overnighting, like Walmarts and whatnot. Not sure why.
If you go to the casino in a big RV, be aware of the low overpass at the exit from I-190. The word is that only one of the lanes is low, but it's one of those curlicue exits so you don't see it coming. We didn't take the RV there so I can't say from personal experience. I know that taking Niagara Falls Blvd west from I-190 is fine in a big RV--we did that in a 40-foot motorhome, towing a car, to get to the bridge to Canada.
The casino is within walking distance of the U.S. side of the falls. But if you've never been there, you definitely should go to the Canada side because that's where you get the "classic" view. Maybe stay until night if they're still illuminating them when you're there.
You can walk over the bridge. Bring your passport, and either 25 cents or 50 cents per person (can't remember, but it was quarters) for the turnstile on the way back from Canada. No fee on the US -> Canada trip.
If you're going there, take I-90, and note that there's a toll for inbound traffic to Grand Island, although EZ Pass works. EZ Pass also works for vehicles going over the Rainbow Bridge to Canada (the one you can walk over).
There are two RV parks on Niagara Falls Blvd within about 10 miles of the falls: Niagara Falls Campground and AA Royal Motel and Campground. They work for big rigs, but satellite might be problematic because of the trees around the perimeter. Neither is a resort by any stretch of the imagination, but not particularly gross or anything (I just drove by them a few times--didn't stay there). You would get to those by taking Niagara Falls Blvd north from I-290. You'd avoid the toll on Grand Island.
Those two parks are expensive, like over $50/night. And actually, I'm not sure how long they stay open. Most campgrounds in New England close after Columbus Day, and I'm not sure if that extends to this part of New York or not, so you should definitely check on that if your trip is after mid-October.
I'll defer to others on your Highway 5 idea. I'm sure a lot of it has to do with the size of your RV. But some sightseeing would be nice--I was pleasantly surprised at how nice Buffalo was.