Hi Paul,
Let me give you a quick overview of the Lower Mainland (ie, Vancouver east to Chilliwack) crossings.
1. The I-5 crossing is the main crossing for all vehicles other than trucks. You just stay on the I-5 as you travel north from Bellingham, then through Blaine although you may not realise it, and then you're at the border--Peace Arch border crossing for the obvious reason that there is a large "peace arch" there.
2. If you keep you eyes peeled as you approach Blaine on the I-5 you will see the signs directing you to the "truck crossing." All trucks must cross there; all other vehicles may cross there.
Both of these crossings, in either direction are busy and the customs and immigration officials attend to their task with due diligence. Coming into Canada the most important thing to avoid is defensive weapons of any kind. They are illegal (without the proper paper work) even for Canadians and citizens of other countries are cut no slack at all.
Try and be close to the limits of what you can bring in. As some one else has said, if you're over but close and if the agent decides you honest and trust worthy on these matters, you're cleared through. For me as a Canadian that means that the sun glasses come off, any hat I may have that may be obscuring my face comes off, I am never flip or cute (although that is very much my nature) and I answer the question asked specifically and briefly, ie, with no gratuitous elaboration. In some fifty years in Canada I have never been searched coming home by car, by RV or in my plane.
3. Further east is the Aldergrove crossing. The exit for this is just as you are coming out of the north end of Bellingham--Belli Fair area, look for signs to Lynden.
This is my preferred crossing although it takes me out of my way for getting home because it is small, busy only on long weekends or other forms of holiday weekends. For example, at Aldergrove going into the US they never seem to be interested in whether I have animals on board--at Peace and the truck crossing, I've been asked almost every time and that invariably precipitates inspectors prowling around inside the coach, and then everything is up for grabs. At Aldergrove, my coach has never been examined. Oh yes, if they are going to have a look in the coach, you are required to leave the coach, well away from it, and they do the examination out side of your vision.
4. Further east of Aldergrove is the Sumas crossing. I've never crossed there in my RV and haven't done so in a standard vehicle in perhaps twenty years, so I've really no useful information on that crossing. My hunch is that it will be similar to Aldergrove in practice.
Since you are on your way to Alaska and I'm guessing that means that you'll be taking the trans-Canada highway east from the Vancouver area, my advice on all counts is to take the Aldergrove crossing; from there go due north on the secondary highway out of the crossing until you arrive at the trans-Canada, and then "hang a right" and go east.
That exhausts my experience of Canadian / US border crossings in my RV--other than one back into Canada in the Kooteneys, but you'll be no where near there, I don't think.
Have a great time in Canada.
Ciao,
Doug