The easy way
I bought an RV-8 project in Ontario and drove it back to the US in a rental truck last November. I used FedEx Customs Brokerage to do the paperwork and haven't regretted it a bit. Total cost was about $180. The thing is, the customs and border people are used to seeing trucking companies go back and forth with the same trucks and the same drivers over and over. Everyone is documented and there are no surprises; that's what the systems expects. All their crossings are automatically pre-arranged, a broker does the same for you. If you drive up in a rental truck they don't know is coming, you're going to stand out. Standing out is bad.
If you use a broker (I'd recommend FedEx, but they may all be similar) you'll do all your paperwork up front: drivers and vehicles will be documented with Drivers Licenses and VIN numbers, contents will be already documented and all the forms will be in the hands of Customs when you drive up to the window. So when they ask you for your paperwork, you'll hand them a bar-coded thing that brings up the entire paperwork package on their system because they have it already. It makes them feel better; it's normal. It took us 5 minutes to clear US Customs coming home and 4 minutes was just looking at the stuff they could see with the back of the truck open.
Also, in the random chance the Canadian Customs is curious why you're driving an empty rental van to Canada (the world situation being what it is, not a totally wrongheaded concern) you've got all this paperwork already prepared for your re-entry to the US.
Yes, they'll come and look in your truck, in my case it was obvious they were just curious. They didn't even get up in the truck to look thru things.
Plus, it kinda QC's your work: If you do everything to make FedEx happy, it guarantees Customs will be happy.
Find a Customs Broker and let them do the work for you.