Well I'll be, the fleetwood sending unit in the Rock Auto catalog has four wires coming out of it! I wonder what the fourth wire is for! And RockAuto says it's only for the '94-96 Fleetwood. Learn something new every day.
Yes, the second ground (black wire w/ white stripe) is a dedicated ground for the fuel gauge, however there is some debate as to how important it is. I encountered it when I did all the upgrades to my fuel system, which in turn led me to learning how to put together weather-pack and metri-pack connectors from scratch plus the acquisition of some new tools for doing so. I kept the dedicated ground, others have combined the two grounds and claimed no ill effect, I maintain that there had to be a reason why they did it that way but to my knowledge no one has definitively solved it either way. On the newest iteration of the racetronix upgrades, they use the normally unused 4th wire (all b-bodies) to run a second dedicated +12v for higher performance applications.
The Fleetwood would have difficulty adapting to this, but I suspect if you knew what you were doing you could run the main ground direct to the sending unit inside, and then from the outside to the factory grounding point. I suspect some welding and/or soldering would be involved. Some vehicles are already grounded that way, especially cars with metal tanks (ours obviously aren't), then you could still have the dedicated ground to the gauge and also have the dual hot wires. Since you're mainly going for a stock or mild build, you won't have to mess with that. Since my build changed I've been debating on going back in and seeing if I can run dual pumps in the tank which would call for a second hot wire at which point I would have to cross that bridge.