Dazmond, I have a gardiners backpack and I think I would be able to clean those windows with one fill.
Depending on what flow you normally use with your van mount, it may take a little getting used to although I find full flow on the back pack too much anyhow. I use 1.4 mm jets now and found that helps me to save water although I think you use an aquadapter so maybe not an issue.
I ditched the backpack straps straight away and hate the screw on lid with handle on it and do not use a trolley because I carry it through terraced houses a lot. I modified mine and made a much more comfortable and fail safe carrying handle ( cost less than a fiver )
I can post a pic if anyone is interested.