Boil potatoes in batches in a canner or other large kettle for 12-15 minutes or until fork tender. I boiled this in 3 batches. Remove potatoes from water and put in large pans (I used roaster size foil pans and put potatoes in a single layer)and cool in refrigerator at least 6 hours. Boil eggs and chill.
Once eggs and potatoes are chilled, peel and chop the eggs and peel and chop the potatoes into bite-size pieces. Using food chopper, finely chop onion. Mix together in large bowl. I use a 30 quart bowl.
In a large bowl, mix whipped salad dressing, mustard,sugar, garlic salt and pepper.
Pour dressing mixture over potato/egg/onion mixture. Put on food service gloves and use your hands to mix together. I use the glove method as I've broken wooden spoons trying to mix it with a spoon!
Put into bowls and chill in refrigerator at least 6 hours. This batch will fill 2 Tupperware fix and mix bowls and a 13 quart stainless steel bowl.I usually make this a day ahead of time so it has time to chill. You may need to add more sugar,garlic salt or pepper. Taste after it has chilled about 4 hours and adjust seasoning accordingly.