That section of Robert Moses doesn't have any low spots nor are there RV restrictions on it. Last fall left Niagara Falls that way in my 10' 7" truck camper but also saw motor homes going the other way.
They do have a large lot for RVs but it is the last one and a good hike to the falls unless you catch the shuttle.
If you have time, I would recommend staying a night or two at Four Mile Creek SP and just running in with your vehicle. From the state park it's a straight shot in on Robert Moses Parkway with very little traffic. And RMP is one of the few parkways I came across in NY where RVs are allowed. It is only listed as no commercial vehicles. You definitely want to stay off the ones that say cars only.