As agri says wait untill it dries then brush it of, it will leave a mark if it is only slight after a few times of rain it should go, if unlucky and the stain is really bad, wash it of with water and a small brush allow to dry and reseal the said area, have only had to do this once many years ago on a unit, thought at the time good job cows can't fly.
Not quite canvas, but I got a sh1t load of it on the car the other day - teach me to park under phone lines. Eventually removed it after 5 mins rubbing with alcohol based window wash from the car. Got another bit on Sunday on the bonnet, used window washer again - taking to long, so decided to spit on it. Hey presto - human saliva removed it in 2 secs. Don't know if you should gob on your trailer tent though.
Hiya All just reading your post.. just a tip on about bird poo on your car dont leave it as it stains. We have 2 moddifed cars sat on our drive that are very heavly polished.. and if for some reason we dont get the bird poo off asap.. It eats through the polish and marks the paint.