I cross the border on a regular (read every weekend in the summer) basis, though I am Canadian so I'm not sure if slightly different rules apply to me.
For your meds, make sure all the bottles are clearly labelled with your name, doc's name, etc so it can be verified if necessary.
For your Yorkies, I imagine it's much the same as me going into the U.S. with them, make sure to have their paperwork (shots) with you to show the border guard.
For food, I don't THINK there's anything you're not allowed to bring into Canada, most of the restrictions were heading south, but I'll attach a website where you can check. If there is anything you're not allowed, don't bring it, or at least declare it. They can seize your vehicles if you omit something and get caught. At the very least they will throw it out, which can get expensive
They may or may not inspect the trailer, depends on the guard. Most likely you'll be there for 30 seconds at the booth where they'll ask where you're from, where you're going, what you have to declare (food, cigarettes, alcohol, purchases) how long you'll be away, the purpose of your visit.
For the Maltese puppy (congratulations!) you'll want to have the health certificate if you plan on declare him upon your return across the border, and his/her paperwork (shots, etc) as well. Also, keep in mind buying your puppy in Canada will be the same thing as buying a tv or stereo to the border guards, so make sure that you're either under the allowed limit for the time you'll be away - I think it's $750/800 for 1 week - or have the breeder make a receipt for a small amount, in case you make other purchases while you're here.
http://www.cbsa-asfc.gc.ca/fpa-apa/law-loi-eng.html
Hope that helps!
Tara