It offers a lot of storage which I like. How easy is it to fill the tank with that thing? Also, I have a  ram mount that bolts above the handlebar nut, for my phone. Is it possible to see a phone with this huge bag on?

I used a magnetic Cortech that worked well. It didn't go above the tank fill, so the plastic didn't matter up front.

I don't have one.

I'll say that up front so you can just skip down to, hopefully, a response from someone who has one.

I've looked at the photo and read the instructions and combined with my (non-Kawasaki ) large tank bag experience I've been embolden to respond.

I don't think you will have any trouble seeing your phone. My large tank bag experience says the only problem you may have is inadvertently honking the horn during full left lock maneuvering.
I use a gas ring mount, so when refueling I always dismount the bike and pull the cord release and set the bag on the seat. It appears that if you normally refuel while sitting astride the bike that
the Kawasaki bag (as would mine) will be problematic. If you have no problem with a dismount to refuel, then it appears that you would need to pull up on the rear to release the hook and loop and then either; remove the front straps from the bike's hooks (if the bag is fully packed), or set it aside the gas cap (still attached to front hooks) if it is lightly loaded. It does look (to me) that it would be more hassle than a gas cap mount but then it is considerably cheaper than most of those, and if you don't think you'll use it much then it's probably a good option. If, like me, you feel a little naked without a tank bag then I'd spend more and go with a bag that has a gas ring mount. Most of those are ugly when the bag isn't on the bike so that may be another consideration for you.

I also do not have the Kawi one. I have a large 3 piece bag. I had the straps but modified it to accept magnets. Pull it and set it on the seat, easy peasy. I only use mine on long trips or when planning to camp. Cant speak for the phone mount. I dont think it would work with mine with the bags full and extended.

Wayne

I want to spend a month next summer traveling the country, so I really want to up my cargo capacity. I have a Shad 50 with the optional luggage rack on top, thought a tank bag would be nice. I'll probably also bring a duffle back to strap on the back of my seat.  Yeah, I know, might be bit top heavy!

I want to spend a month next summer traveling the country, so I really want to up my cargo capacity. I have a Shad 50 with the optional luggage rack on top, thought a tank bag would be nice. I'll probably also bring a duffle back to strap on the back of my seat.  Yeah, I know, might be bit top heavy!

Yeah a good sized dry bag on the passenger seat can hold a surprising amount of clothing. I use gallon and two gallon zip lock bags to keep it organized inside. You can easily go 10-14 days between laundry days that way. Maybe longer if you don't mind a little stink here and there.

This holds a ton of stuff, and makes for a good backrest.

This holds a ton of stuff, and makes for a good backrest.

Hard to imagine that would fit with a Shad 50.

Hard to imagine that would fit with a Shad 50.

Oh, I think it will fit. It's dimensions are smaller than many passengers that I see on the pillion stuffed in between the front rider and top case.
It won't feel as warm and squishy on the back, but it'll be considerably quieter.