Trundle is an underplayed sleeper champion that has various ways to build him. Some work, some,not so much. Although I have figured out a way that you can master Trundle jungle

Runes consist of:
Attack damage marks
Armor seals
Magic resist glyphs
Attack speed quints

Try doing 9/21/0 in physical damage and armor and magic resist for more of a tanks build, if your team is packing damage, but are relatively squishy, but if your team isn't packing damage, go 21/9/0 and take all physical damage Masteries except for critical Masteries, and take a few armor and magic resist runes.

Start with the basic jungle items: hunters and 5 pot
A first recall, go for wriggler's lantern, (unless if you want the attack speed, its still viable, but if you want more cc and damage, take Spirit of the Lizard Elder.) and berserkers greaves
After early game, go for attack speed items, such as Blade of the Ruin King, and Wit's End
Lastly, finish off your build with spirit visage, and zephyr

Start off taking your q, then your w for better sustain. Hold your e until level 4, since it won't be used in the jungle. Max your q, then w, then e, and your ult whenever possible

Pros:
TONS of sustain
Incredibly tanks
Hard to kill after full build
Destroys squishy targets, and can widdle tanks health
Can save teammates and self with w and e if being chased
Can push towers quickly and with ease
Cons:
No hard cc
No great initiation
Can be killed with hard cc such as stuns, roots, suppresses, etc.
No good mid game damage
Can't counter jungle buffs
Ganks are hard to master

When jungling, make sure you last hit the last minion of the camp with your q, so your power isn't reset while moving from camp to camp. Also start with your w when you fight a camp. The sustain, and defense can keep you in the fight
Taking big camps such as baron or dragon are easier with q, (which takes away damage from the target, and is gifted to you,)and lifesteal you get from items and your w.

Clearing minion waves are a snap with trundle, since you can take more power from minions, use your w for faster attack speed and more lifesteal. Last hitting with your q doesn't apply as well as it does with clearing jungle camps, although, it does reset your attack timer, you if you miss a minion, you can press q to make sure you can get the minion. Your q also works
On towers, but unfortunately, it does not give extra damage, so use it when pushing a lane faster.

As trundle, make sure you are the one dealing damage, as well as taking it.
Focus the high priority squishy targets, such a d the adc, apc, or the support. But be careful of hard cc. You will be killed. Also make sure you have your team with you if you try and assassinate champions. When you ult, make sure its on the tank. He will lose armor and magic resist, and it will be funded to you, also he will be deprived if health and you will be gifted with it.