One bad plug means something is wrong on that one cylinder. It could also cause the economy drop, but it is possible the bad plug and fuel economy drop are unrelated (bad tank of gas, etc).
I suggest start with 2 simple cheap things:
1) Run a bottle of Seafoam or equivalent through the gas. Let the level go down to 1/4 tank or so, pour in the whole bottle, and drive normally. It is great cheap cleaner for the fuel system.
2)Pour a bottle of auto-RX into the oil, drive 500-1000 miles, then change to a nice oil if you are not already (Mobile 1 is a safe bet and there are coupons to make it cheap)
The Seafoam treatment has solved many a minor issue on my cars (works great on small engines too, lawnmowers and such). Lumpy idle, difficulty hot starting, etc.
The AutoRX treatment is a good cleaner for the oil system. If you have the time and ability, an alternative is to pop off the valve cover and inspect for sludge buildup. Using high quality oil will result in little to no sludge.
I'm a big fan of cheap/easy chemical cleaners, especially on cars (like yours) that do a lot of sitting.
One instance:
Some years ago, DW's car was smoky on startup. It would burn a little oil, then clear right up after 10sec or so. Compression test, etc was fine. Other owners on the forum had similar issues and were starting to panic: rebuilding heads, entire engines, etc. It turned out that the (very delicate) oil control rings had enough gunk built up they would not seal 100% on cold startup. An ounce of Seafoam in each cylinder, rotate the engine gently by hand, let it sit for a a few hours, and the issue solved.