I have the same Tusk ATV tank bags. They are ok for the money. With the stock tank they aren't secured very well since the gas cap isn't as large as an ATV gas cap, or probably the Clarke's gas cap. I didn't use the little velcro strips either, because I didn't want to stick them onto the tank. I tried off-roading on some steep, rough terrain and the bags cam off the gas cap & slipped forward onto the steering column a number of times.
I have had better results, especially when they're empty, placing the tank bags across the seat and sitting on top of it. But that doesn't work very well either off-road because you need to stand up quite a bit.
They aren't the best bags around, but they are well worth the $20. I find that they are handy and add a good bit of capacity along with my other "cheap bags", the Mountain Smith large "dog bags" that are about $38-$40. I highly recommend the dog bags for the price.