I need to replace the tires on my Yacht Club trailer and they are bias. I am wonder if
I can go to a radial instead.

Another question I have is what causes a really bad cupping to happen on the inside/outside
of the tire?

Any help on these tow questions is much appreciated.

Are bias ply trailer tires made anymore? Radials are fine.
The cupping is likely caused by underinflation or a really bad alignment.

Go radial all the way. You can still get bias ply trailer tires, but why?

If they are cuped on both the inside and outside, that would indicate underinflation. If on ly one side or the other would suggest alignment is required.

Get the radials, and inflate to max marked on the tire.