We generally use the product we use for car washing - something like Turtle Wax car wash or similar. If you have some stubborn spots, suggest you don't use a Magic Eraser, but try a bit of Simple Green and rinse very thoroughly. We get a product at Dollar Tree, LA's Awesome - a yellow colored product in a nozzle bottle - it says to dilute it per...... DH, not always one to read instructions has used it full strength on the bad spots and they're gone. But he has the canvas wet, sprays this on, gives it a soft scrub with a sponge or towel and rinses very thoroughly. This is not a recommendation, mearly something has worked for us, so try an out-of-site spot before you get into this. We have never used the 303, but have heard wonderful reports. If you find the inside needs a bit of freshening, I use Woolite and then a towel to dry as much as possible. Be sure to leave the bunkend open until everything is completely dry - maybe a day or two.
If you normally use Dawn, I'd see no problem. If it's safe for baby ducks after an oil spill, it should be good for your bunkends.