For support I would build him pure tank runes and masteries, for top lane I would look at AD or ArmPen Reds, AD or Movement Speed Quints (maybe a mix), then choosing between flat/scaling armor yellows and magic resist blues depending on your opponent.
Top Lane - Besides your ult max Q>W>E
Support - Besides your ult max E>W>Q
Top Lane - I like getting a Tiamat on my first back if I can, then the rest depends on the opposing teams comp. If they have a good bit of AA damage and I am facing AD damage I will rush Ninja Tabi>Iceborn Gauntlet\Frozen Heart>Spirit Visage (Spirit Visage has good synergy with Trundle' passive) then upgrading Tiamat to Titantic Hydra when my initial tank items are complete.
If I am facing magic damage the build will be similar, starting with Tiamat but then rush Spirit Visage first.
As for other items they are situational depending on who you are facing. I would keep ZZ'Rot Portal, Guardian Angel, Dead Man's Plate, Banshee's Veil, and Randuin's Omen in mind.
Support - Eye of Equinox>Boots (your choice, I like swiftys\mobis on support)>Iceborn Gauntlet\Frozen Heart>Lock of the Iron Solari>=Spirit Visage.
Then rounding the last item out with Dead Man's Plate, Randuin's Omen, ZZ'Rot Portal, Guardian Angel, Maw of Malmortius, or Titanic Hydra, again all depending on how the game is going and what the enemy team comp looks like.