One more opinion here FOR throwing out the 3-way and getting a compressor (we have an 51qt IndelB box, on a slideout, at the spot where the FWC icebox was, i.e. the original freezer spot). They're just better than that old technology, and you'll always have sun or a driving car nearby, while propane might be hard to refill in some places.
90W Solar should do it, if you're going around in summer/sunshine, AND your batteries are also charged by the truck when it's running. Get a CTEK Dual, seriously. It's a 3-bolt solution that charges with 15-20A, that's about as much as 200W of Solar will do under non-perfect conditions (i.e. everytime - sun angle, temp rising, etc). It has an input from the truck's battery + and auto-disconnects and autocharges. It should do it as long as you're no longer than maybe 3-4 days on one spot and sun doesn't show up (you can do more more elaborate calculations, and also please tell us what else you run off the battery - how about laptops or other goodies?
How is your 200Ah battery charged at the moment? Make sure you have a good charger, otherwise you're driving around lead ballast without any use. The CTEK is such a thing... and by far the easiest setup for a dual battery with alternator+solar charging.
If I were you, I'd get one of the already mentioned flexible Renogy to add another 100W. They're 200$, we have one on our FWC and it works perfectly. You already have solar wired, so it's more than easy. It just adds a little peace of mind that you'll be able to trad ea cold beer for fish even on the 5th day of standing on that beautiful beach.
I now have our panel on a DIY-wind fairing based on Thule racks (and chair+table storage underneath), but for about 9 months, I did a temporary mount (since I knew the fairing would come on day): just use the outdoor 3M foam mount tape (the red one). I had a total of about 15 inch (3/4" broad or so) for the panel mounted on the truck roof, and it was a serious PITA to get it off again. No truck on earth has a top speed where this panel would have gone off.
It shouldn't be less able on the FWC roof. Use more tape (or get the 3M 4991, that's the stuff that GoPro gives you for camera mounting), and you'll tear apart the camper before tearing off the panel. Please don't drill, it's just not necessary with glue/bonding technology nowadays, and it will just give you leaking. If not now, then later some time.
For gas mounts: I have a friend who mounted two Jerry can holders to the back of his Hawk. A good time to do this is when you have the fridge out, since you can add big washers and 3M 5200 on the inside.
I would trust glueing stuff down (our vanagon back in Europe has a solar panel and a LED bar glued to the top, Sikaflex 252), but I wouldn't trust the exterior aluminum of the FWC to hold it.
Goodluck!
Best,
Michael