Mundo doesn't really shine in late game, sadly. I think his torch is pretty bright in Twisted Treeline, and if he were better in 5v5 he'd be overpowering there. A few tips:
-Tanks are widely thought to be bad place, every one of them. Last Whisper and Void Staff are super common, damage items are more slot efficient than tank items, super late game tanks get blown up basically no matter what. This is a problem for tanks that can control the terms of engagement like Nautilus or Malphite, and a really big problem for tanks who can't initiate like Mundo.
-I run MPen marks. Helps with cleaver, helps with fire, helps with Sunfire
-Despite the above tip, Masochism does massive damage. At max rank, it adds 100 damage to your autoattacks even at full HP. Definitely worth hitting people with, even if you don't build to help it at all. Maybe building some attack speed or armor pen would be worthwhile to synergize with this: I doubt it, mpen marks and Sunfire are good because they don't distract from massive tankiness.
-Sunfire Cape>Warmogs. I think you'll find that, in many teamfights, the enemies ignore you to kill people who die faster. This helps you punish them for that without sacrificing tanks. New Warmogs is an anti-poke item, and not really what Mundo needs.
-Spirit Visage>everything. Glad you figured that out.
-Randuin's Omen>Thornmail. Almost as much armor, tasty tasty HP, and a form of CC. Thornmail's nice, but if you're going to get it, get it last.
-Mundo bait. It's hard to tell the difference between "I can let them dive me, ult away to survive and my team will kill them while they're overextended" and "if I get hit with CC, I'm instantly melted :(" but that's the difference between an alright mundo and a really solid one. Make sure you know, not only where the other team is, but where yours is: going in alone is pretty pointless. Honestly, Mundo's happy throwing cleavers and letting someone else engage. If you hit someone with a cleaver and they hit you with their poke, Mundo usually wins that trade.
-If the enemy team is focusing you, one of two things happened: either you went in without your team behind you, or you're doing your job. Sometimes, dying is part of doing your job. If your team is following you in and still losing the fight, they're not doing their job. If your team is nearby, you go in, and they leave you to die, you're probably the weaker team at the moment (or your teammates think you are) and they're too scared to engage. This makes Mundo very sad, but all you can do is stick with them, wait for the enemy to engage and activate tank mode.
-If the enemy team is ignoring you, pick someone squishy (ADC 75+% of the time), stand next to them, be on fire, pop Randuin's, make many cleavers, block skillshots, get Masochistic, and ult so you can do it some more. Often, if the enemy ADC kites you away from the fight and takes forever to kill you, that's actually worthwhile.
-Ignite>Mundo. Sad but true. Probably not worth building QSS to counter. Very sad, probably still true.