We parked on the US side last month at the closest State Park lot to the Visitor Center, Lot #1. Two tips: pay close attention and don't get lured by hawkers into other lots you will pass on the way. You want the state park's lot, so look for State Park Parking signs.
Second tip, get there early, by nine a.m. or sooner. Shorter lines earlier anyway for Maid of the Mist, etc.
We parked at the end of the lot by the shady sidewalk and backed in, no problem. Still cool inside hours later. A nice woman in a short Sprinter Roadtrek came and backed in next to us. We returned the next day and parked in same location. Gotta love a Class B!
Yes, the park operates a green trolley continuously from point to point inside the park, not the town...so just leave the B and walk or ride over to Goat Island for Cave of the Winds, restaurant,etc. The cost is $2, but with the Discovery Pass (recommended), you get one trolley day free.
The pass includes all the attractions on the US side. The state is pumping tons of money into renovating all the facilities, and the flowerbeds, walkways etc. all looked great. Staff was friendly. Food was good at Tops of the Falls...overall, a very good experience for us.
Very easy walk from that #1 lot to the Rainbow Bridge to Canada side. You can walk over, take pictures, have coffee or lunch, etc, and walk back. Immigration very quick on both sides. As you stroll, you can take some great photos from the bridge, while dozens of drivers sit there twiddling their thumbs awaiting the clearance to drive into Canada.
If you camp at Four Mile Creek State Park 15 minutes north on Lake Ontario, your parking at the falls state park lot will be free. Otherwise, (I think) it is $10 a day.