I noticed them at Costco the other day as well. They had the Kirkland group 27 size deep cycle battery for $89.99.
Will check the amp hour next time I'm there.
Here is an option with a pole, http://www.bowstep.com/. Not sure if/where they are available in Canada. You may be able to fabricate something similar.
Once you hook anything up to your hitch it is treated as a single vehicle for insurance purposes. That is why if you tow your boat/RV make sure you are fully insured (optional loss or damage coverage).