New single axle trailer with medium heavy 18 foot boat on it with radial tires on it. The tires only have maybe 5k on them and they are getting bald on the inside of the tread any ideas? It tracks down the road good ,camber and toe look okay? Maybe to light of axle?

First thing I'd do is get your rig weighed, so you can establish a baseline of what your actual load really is. Compare this to the tire and axle weight ratings, and see what you have. Since you mentioned that it's new, once you know your load, start talking to your dealer, and see if too light of a trailer was put under your hull. Inside tire wear is usually caused by too much negative camber (top of the tires leaning toward the centerline), and may be a result of too much weight on a under-capacity axle/spindle/tire combination.
HRG

This happens to me about every other year from rough roads. It sounds like your trailer axle is out of alignment. I pay about $50 to have mine aligned. The first year I had this happen I called the dealer and he told me they are NOT aligned from the factory. But yes, also make sure you are within your load limit.

Tapped, My previous trailer gave me this problem. The trailer axle was basically 2000#'s and the boat and motor weighed atleast 1750. Then adding the kicker motor, trolling motor, 35 gals of fuel, fishing gear, etc, etc. i'm sure it was well beyond that weight. I took it to an axle shop and they said they needed to straighten it. Once it is out of wack it will not last. I solved the problem by having the axle replaced with a 3500# axle, two new tires and no more problems. Hope this helps. lou