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Fuel heater, GFI?
I think you've got this mixed-up. The 110v plug your GFI controls has nothing to do with your fuel heater. It is a "block heater" located above the oil filter. If it's really cold this will assist in cold starting if your glowPlug system is working OK. If your GPs are NOT working the block heater becomes a necessity (if it's ZERO degrees and your block heater is NOT working AND you can get it started at all, your GP system IS working
The fuel heater is located in the bottom of the filter housing, under the plastic cover, at the top/front of your engine. It doesn't really assist in starting but is more to help with fuel gelling / water freezing in the filter. If you blow fuse #30 over & over again it's shorted and will prevent the PCM from powering and will never start at all. You'll know the PCM is out because you won't have a WTS light.
Speaking of the WTS,,, when the light goes out it does Not mean the GPs are off, it's just a timer. So, don't "cycle" the key on/off to keep the light lit, simply leave the key ON past when the light goes out when it's really cold (maybe 1 min)
Batteries, charge them up full and the next morning when you goto start put a volt meter you can read through the windshield on either one and watch the voltage go down as you crank. See what it drops to, if it's down around 11v they're tired (make sure your terminals are scraped/cleaned/greased/tight all around for a proper test)
If they're the original batteries, you need new by now. If not how old are they?