I haven't had any issues from my single-axle RoadRunner trailer for my 05' VLX. It's a HD, 5,000lb axle. For the tandoms, I don't know if they just use two of them or two smaller ones.
That doesn't have anything to do with your issue though. But I wouldn't think you should be worried about the trailer not being strong enough if mine works for the same size boat and is only single-axle.
I had some questions about my RoadRunner trailer once and got some good feedback from Kent at RoadRunner. His email is: firstname.lastname@example.org