I love jungling with Trundle.
Start Q machete 5 pots, go Wolves-Blue-Wraiths-Red-Golems-Wolves-Wraiths. Work towards either tenacity boots or boots5. Get wriggles.
Level QWQER if you don't need to gank too hard early, level QWEQ if you need an early gank. Max max Q first. Use Contaminate to boost aspd and movement after a fight starts, and try to make sure you don't screw teammates with Pillar. Trundle's not a killy assassin jungle, he's a tanky annoying chaser. You're best suited to set up kills for your teammates while picking off wounded squishies when you can. Your Q steals damage, use it on any unit to keep it charged, it's like a free 40 attack damage all the time. The debuff is okay and doesn't hurt at all.
I get wriggles, then work towards either Randuins or Mallet. Randuins tends to help earlier on, Mallet if you're actually being aggressive and chasing down squishies. Warmogs never hurts either. Remember, tanky. Your passive will activate with any enemy unit, whether neutral or on the other team; it does not activate for your team.
DISCLAIMER i'm not pro