So heres the deal. I find that trundle is very strong early-mid game, but if the game runs too long I feel like I completely fall off a cliff in terms of survivability and damage.
Its possible that I'm not building him right past the first 3-4 items.
My builds vary based on enemy picks, but I usually try to grab some tenacity, and wriggly's lantern so I can babysit enemy red buff. The variance comes depending on whether I need health, MR, or more damage. I usually try and go with trinity unless i really need the extra hp and then i'll grab frozen mallet instead.
Drop Trinity. It was core in s2, but I feel S3 he gains much more from IBG. Sheen was the main reason Trinity was so good on him, but you already get a small slow on your Q, IBG slows as well. I suppose Aegis and/or Locket make great buys on him, as does Spirit Visage when you want a bit more MR. New Trundle will also really appreciate BoRK.
I'm not sure about what to upgrade Machete into. Wriggle's works well with his Q acting as an auto attack reset, but both spirit stone upgrades are appealing as well.
If you get Madred's, never, ever upgrade it to Wriggle's. Not worth it these days.
As far as items go, I personally love Spirit of the Ancient Golem + Ninja Tabi combo, simply because of how awesome Ninja Tabi is. But it solely depends on enemy team-composition. Madred's is really good for early dragons as well which Trundle is more than capable of solo-ing after level 6.
Good items to consider: Ravenous Hydra for sustain+damage, Randuin's Omen, Spirit Visage, (early) Locket of the Iron Solari, (Runic Bulwark) Aegis of the Legion, Iceborn Gauntlet.
What I usually go for is: SotAG, Tabi, Locket, Hydra, SV, Omen
Remember to use your ultimate on the proper targets, it'll make it insanely tanky!