I agree about ditching that fuel and geeting the tank clean and free of any water that collects down in the bottom...
You can drain the tank down prior by using a 5 gallon gas can, and pull the fuel line off the petcock, slip a 2foot section of hose on, and drain it down using the PRIME position to lighten the tank. This will also allow you to inspect petcock function, and switch repeatedly to flush it out also at the same time. Then remove it , tilt it sidewayss and complete the draining. When that's done, use a turkey baster with a hose attached, to reach those lower wings, to suck out everything there, followed by a wire coathanger with rags attached, to sop any remaining water laden fuel from thos nether regions, swabbing them well, you'll get a visible amount of fine rust cleaned out that way. Flush the tank a bit with a small amount of clean fuel, and dump that totally. Reinstall, and add about 2 gallons of FRESH fuel, from a NEW CLEAN gascan, and add about a hlaf bottle of IsoHeet.
Ride the bike and use that fuel up, and refuel with gas from a different station, one that lives on high ground, and sells lotsa fuel...
As for your milage losses, cleaning your airfilter, and adjusting valves, will drastically improve milage... its likely time.
The dealer should not have had to remove the tank for a solenoid replacement, but everytime a tank is removed, and shook up, and replaced, there is a higher incidence of contaminate laden fuel issues immediatly following...just a fact.
Please edit your original post, to remove the duplicate post it contains, thanks in advance.