If you want the electronics, the RKA is a slamdunk, either prebuilt-to-your specs, or DIY.
The straps DO NOT scratch your tank unless you're doing it wrong. To re-fuel, it's only necessary to loosen or unsnap one strap and slide the bag over.
It's dirt simple to switch the bag between various bikes you use (say "click" three times).
The mapcase is large enough to handle a full section of most road maps without additional folding, and rips right off to take inside to study over a cup of coffee. Mounting the RAM on the front of the bag adds a degree of shock-absorption to prolong the GPS' life. When removing your expensive stuff for security's sake, it's only one wire to unplug for the entire set-up -- all the rats's nest of wiring is compacted inside the bag, and once configured doesn't ever need untangling. Extra lead out of the bag for yuour electric vest? It's right there. Cellphone charger? It's in there, and connected to your electronics or not -- your choice.