Nitrates and phosphates do not get trapped in the sand. The water containing nitrates and phosphates does, and it will eventually seep out into your main aquarium. However, it will not be in an amount greater than using tap water to do an emergency top-off, which is not a disaster.
If you use just tap water, then you will be adding a small amount of water with some nitrate and phosphate content, but unless the levels in your tap water are through the roof, even 1-2 gallons of tap water isn't going to matter that much in a 55-gallon aquarium. Test the water and divide the level by 25-40. That will be your final amount in the aquarium as long as you fill the tank with RO after the tap water rinse.
If you follow the tap water rinse with an RO rinse before adding it to the aquarium, you will be adding no measurable nitrate or phosphorous to your aquarium.
A lot of this hobby is governed by superstition and "popular belief" and that's what makes sharing stories so interesting. But there are some things that just aren't backed up by science. Properly drained, each 20-pound bag of sand when wet will contain about 0.67 gallons of water. 60 pounds of sand will contain about 2 gallons of water. A single RO rinse after a tap water cleaning will have no difference in NO3 or phosphate levels than just rinsing in RO water.