Just a couple of things that can make the whole thing much easier.
Respect. Yes sir/ma'am, No sir/ma'am. Talking to them respectfully goes a long way in making the entire process easier and quicker.
KNOW what you have and what you don't have. If they ask you about something and you are obviously not sure of the answer, they can put you aside and go through your unit. They need to be sure of your answer and, if you can't give them confidence in your answer, they'll do it themselves. What would really help is a notepad with what you have in the RV written on it. It's not cheating to look at it, and they will see that you took the time to inventory your RV - it helps you and them. At first it might seem like a daunting task, but I'd be surprised if this took you more than just a couple of minutes prior to arriving at the border to account for all food (including pet) and beverage you have. Since you're likely to make a stop for gas before hitting the border, do it then - it'll be the most recent inventory you could have. If there is something on it you can't bring, they'll take it away and that's it. It's not a black mark and they won't berate you!
Being from the US, it's extremely likely that they will ask you about a gun... turn to your partner and say something like "You did take the gun out before we left, right?"... well, they'll probably have a look themselves, since you're not inspiring a lot of confidence about that answer. Same thing with food, tobacco and alcohol... again KNOW what is there and how much (specifically for alcohol and smokes). Just don't lie - everybody has 'tells' and they're trained to find them. A well-trained officer is just as good as bring hooked up to an actual lie detector.
It's not a hard thing and is no reason to stress over. Quite frankly, the only people who need to stress about it are people who are doing something wrong. You're not doing anything wrong, so you'll spend 30 seconds or so talking to some guy/gal you've never met before and be on your way. It's when they see someone stressing about it that they'll need to wonder if there is something up.
Another thing... they're not your buddy or your pal. They're not chief or boss or any other friendly kind of nickname you might be used to calling people. They're not there to be your friend or to listen to jokes or wise crack answers. These things can be taken the wrong way. Actually, it's one of the things that public servants are taught to be on the lookout for while interviewing (yes, it's an interview). Be straight-up with your answers - majority of them will be yes or no answers and that's all you give them. You give them no reason to doubt your answers and they won't. The driver is being spoken to unless indicated otherwise, so the driver should be answering unless indicated otherwise.
Keep in mind, it is always possible that they can pull you aside regardless and do some kind of search. This can be a random thing and nothing to do with suspicion, you or your answers. I've had this happen on both sides - no big deal. They'll either do a quick physical search or pass by with some kind of x-ray truck (they'll ask you to get out with your pets, if any) and it might delay you all of 10 minutes. Again... nothing to stress over.