In the field, one solution might be to wipe the zipper down on a regular basis with a damp cloth. Do that when the wind is not blowing, of courseotherwise the moisture will just attract more dirt. And I'm afraid it's a bad idea to spray ANYTHING on a zipper, including silicon. That's fine for wetsuit zippers, but awful on outdoor-gear zippers that will be around dirt.
What I would do is this: Fill a bucket with some warm water and add to it some Dawn detergent (a good emulsifier for grease and oils). Get an old toothbrush, dip it in the bucket, and got to work on the zippers. Lather them up pretty well with the soapy brush. Then rinse them thoroughly with clean water (the soap residue will catch dirt) and let them dry. Then, rub some paraffin wax on them. Just a littlenot much at all.
If all else fails, zippers are fairly easy to replace. A good outdoor-gear repair outfit will be able to remove the old zips and sew in new ones, and the tent will be as good as new.