Tires: Trailer tires

Question
I have an enclosed trailer rated at12000#s.  While I haven't weighed load, I estimate total trailer including load weighing at close to 8000#s.  The machine it carries weighs in at 4500#s.  The tires I've been using are uniroyals lt245 75r16.  I use the trailer at least a couple days a week and when I use it there's a couple of hours of highway travel and then a lot of city driving. Which in the New Orleans area, can be rough on trailer tires. ( countless potholes, curbs and narrow streets).  
My problem has been getting tires to last more than a year.  Always seeming to slip a belt or blowing out.  I notice that on elevated roads the trailer will bounce (oscillate).  Which for now I'm trying to avoid, not always possible.  I'm trying to decide on some of the steel encased lt tires or an all steel trailer tire.  Lt's are rated at both 10 and 14 ply and the st is rated at 14 ply.  I'm assuming the bouncing is creating heat build up and the city driving is adding to the issue.  Just tired of replacing $200+ tires and changing tires after a long day.

Answer
Harvey,

So let's do the math.

An LT245/75R16 Load Range E (10 Ply Rating) has a load carrying capacity of 3042# at 80 psi - and I'm assuming the trailer has 4 tires, so the total load carrying capacity is 12,168#.  I don't think tires ought to be load to more than 85% of their rated capacity, so the trailer can not weigh more than 10.343#.  Plus if I factor the fact that there is side to side and front to rear load variation (I've estimated as much as 15%, then the trailer is effectively limited to 8.791#.

You said the trailer weighs 8K and the load is 4.5K and if I estimate the tongue load to be 15%, then the load on the tires is 10,625#.  I think you can see the problem.

Let try this with an LT235/85R16 Load Range G (14 Ply Rating) - max load 3750 @ 110 psi.  I'll skip the math and say that the math works - barely.

The problem here is that you wheels have to be capable of handling 110 psi.

Another problem is that these are commercial tires and you'll have to work with a commercial tire dealer.  The only tire I am aware in an LT235/85R16 Load Range G is a Goodyear G614 RST.  

But one thing I don't know is how things are around New Orleans.  A local commercial tire dealer will, so I would find a reputable one, and work with him.
So I would work with a commercial tire dealer