I made an short guide about Trundles items when i jungle him. I always jungle until level 18 because i love ganking people.

for the last item you should look if you need armor penetration(Last Whisper, Black Cleaver) or if you want damage then take Bloodthirster. Of course you can also buy armor or life items and you have a game where most opponents(4-5) are ability power you should buy Force Nature after Trinity Force. Often the game wont be so long so you cant buy all items.

In the jungle first kill the big wolf and then take an Heal and kill the Mana Golem with Smite and Spamming Q.
When you finish kill the 2 wolves an then the 4 ghosts and the 2 golems. Now you should go back and buy Longsword for Madreds Razor. Now kill the Redbuff with smite. Now you can look for a gank or jungle longe.
When you go in jungle at the beginning you should look in a bush if they want to gank you and make firstblood(happend to me one time) but now i always look where they walk.

If you dont want to use Exhaust you can also youst ghost but i prefer Exhaust because you are fast Enough with Contaminate and its very nice in combination with Pillar of Filth.

I made the masteries in Defense because it is safer in Jungle and while ganking.