Hi Katie,
I do think the border agents look a little more closely at a motor home than a car.
We've crossed the Canadian border from the US a few times. It's more difficult coming into the States that going into Canada.
WE (US) are more strict regarding certain food stuffs. What has worked really well for us, when we are in Canada, is to call the Point of Entry of the US where we plan to cross the border to get the list of restrictions. The number one thing you'll need to do is make sure you don't bring certain foods into the US. That restriction changes daily it seems. So we call, get the list of foods that are not permitted into the States and begin to "clean the fridge". You'll also need current health papers for any animals you bring across and a passport.
Although we felt we didn't get a lot of info from web sites, if you goggle "entering the US", you'll find websites that list restrictions.
One thing you might check is to see if your health insurance is valid in the US. Most Canadians have to buy coverage for the US. And you might want to check to see if you need special insurance for the RV. It is probably covered by the RV rental company; but I would at least see if you have the paperwork necessary. When we cross into Canada, we always have an insurance card provided by our RV carrier to prove we have coverage.
One thought. We have been in the Niagara FAlls areas on a couple of occasions. It is VERY busy and not conducive to any size RV. Parking is very tight and restrictive. Both times, we parked the RV outside and drove our tow car into Niagara FAlls. So if you are planning on actually visiting the Falls area in an RV, beware.
Marsha~