Wet & Forget
If you have plenty of time at least 2-3 weeks
This product naturally removes dirt and stains over time and with the weather. However you have to place your tent in an open area as the dirt is removed with the rain and wind and it can react with any new stains such as bird poo so needs to be away from trees.
Ultramar Power Cleaner
Fast and effective canvas cleaner
This is a great fast acting product to remove stains and dirt from the canvas of your tent or awnings, spray it on, wait and then wash it off.