A few corrections / clarifications & advice from someone who crosses back and forth to the US a lot:
1. You will NOT be pulled over and fined on the 407 for not having a transponder. You MAY be billed after the fact if your state has an agreement to pay tolls. It is a great way to bypass Toronto albeit can be a few bucks :)
2. Don't confuse Customs with Agriculture officers. Agriculture officers in the US & Canada may and have with us - come into our trailer and confiscated certain food items. They change from time to time, but general rule is no meat, citrus fruit will be confiscated unless it has a sticker showing country of origin and that country is on the ok list. Last time we got cherry tomatoes taken away. Be friendly, polite and cooperative and you will RARELY have a problem. We never have. If you have something like fruit you are not supposed to - let them dispose of it and don't worry. It is designed to stop the spread of diseases in fruits.
3. Be aware as others have said about what you can't bring - ie: handguns, mace, tazers, etc.
4. When approaching the customs booth - Turn your radio and cell phone off, put your windows down - drivers and the left passenger one if tinted. Take off any sunglasses or hat. Have your passports ready to hand to the officer. Be polite. Answer questions honestly but briefly.
Be friendly but not over friendly. Be prepared to tell them where you are going, why and for how long.
5. Most times they won't search RV's - but if they do - be friendly and cooperative. Don't try to hide anything you have. If they ask you and you say no, and then they find it - you are going to be in trouble. If they ask and you tell them - you won't be in trouble - worst is you might lose your grapefruit :)
I have used these tips and have NEVER had anything but good experiences with both US and Canada customs at many crossings.
Even with a not so good mood officer - your attitude can make a HUGE difference.
...almost forgot - as with Canadian Credit cards in the U.S. you might not be able to pay at the pump with them, but can pay inside.