Wow! Never had a lower crankout window that didn't close air tight and all our RV's have them as it one of our requirements also. Had the slider type windows on our MH's nad hated them! One of the many reasons we use storms on windows rather than ordering DualPanes. Most all the DualPanes we've seen on RV's are slider windows or not opening windows. Not for us, yuck! If it's an Hehr Window, the rubber around the glass is replacable unless it's some special size made only for a model or two perhaps.
Anyway, I would expect that your sides of the crankout lowers have more gap near the bottom than at the top as the window must be at a slight angle being cocked open a little. Unless the rubber it's supposed to contact to seal is bad. You'd likely need something tapered to seal the gap without putting a lot of pressure on the glass. Your taping sounds good and also taping the Reflectix in place makes good sense.
Another possibility is to buy some simple foam weather stripping 17' rolls at about any hardware store or big box store and NOT take the adhesive cover off the sticky side so it's very easily removable when at home. Comes in 3/16, 1/4, 3/8, etc thicknesses and it real cheap. It will fill the gap and held in place with the glass pressure on it and will be self compressing to varying gap size. Can still use the Reflectix also and I sure would. If I was where you are, I'd be very happy to help you with the problem.