Having the vet draw urine directly from Maggie's bladder will give you a more accurate result...although with the blizzard, it doesn't sound like heading out right now is ideal! If they don't feel well and/or associate the litter box with that, it certainly can cause the aversion.
Have you tried Cat Attract litter? I've heard that works really well. Have you recently switched litters? If you have, that may be an issue...kitties can be fussy about such things. Or, if she has neuropathy, the type of litter you are currently using may be bothering her feet.
And yes, at least for now, setting up several boxes around the house may help, too.