We're getting some significant steering play and wandering on my 99 353 at 50+ mph. Boat has has Latham full hydaulic to the dash, factory ext boxes and tie bar. Everything seems to be tight but obviously not. Any suggestions on where to start looking?

check all connections and make sure theirs no leak,,look under the dash also..if theirs a leak their will be air in the system which will cause loose steering.

No leaks that I can see - is there any way to tell if there is air in the system and how do you get it out??

If it's a distinct zero-effort freeplay with distinct stopping points, it's more likely to be in the helm or the tilt mechanism. If it's a mushy freeplay, it's most likely air between the helm and the rams. Check with Latham on how to bleed those lines.

Gimbals OK? They're a likely cause as well.

Get on the Latham website 'lathammarine.com' and go to FAQ's. There are instructions on how to bleed the system. I would get it fixed right away as it would be very dangerous to loose your steering at a high speed.