Aside from floor plan I would choose the one with the best Chassis condition. I bought a 1999 Southwind in 2006 and over the next four years spent most of the money on Chassis related expenses. In particular brake lines.
Just so you do not get in to trouble and have a bad experience. You can only use the 407, if your total weight is less than 5 tons. Other wise you will require a Transponder, which for a one time use is not feasible. I will save you my tirade on this subject, which I have already send to the 407ETR.
This topic gets more misinformation than any other. The 183 day rule is an IRA rule and you can send in a form that states that you do not intend to become a resident of the US and that all of your holdings and finance are in Canada. Immigration controls entry into the US. I have had friends who spent up to 6 months every year in the US and have no issues with gaining entry. You just need to get all of your paper work right.
You could use a new tote tank of the size you need to carry the water. No opinion on whether it is a good idea or not. Must admit a number of my brain waves were ahead of their time. Maybe they would work in an alternate universe.
For a start there is a nice campground right in Toronto called Rouge Valley. Good place to start and get familiar with the systems. It is also close to the Toronto Zoo and to the GO Transit to get downtown.
You can do both. Hurray one way and take it easy the other. Lots of things to do on the southern Ontario portion. If in a hurry you can cut off a lot of time by taking the Ferry at Tobermory over to 17.