I'd agree with what's already been said: check the original connections, if they were just pigtailed & taped then you could lose significant voltage across the corrosion - or the black box could be bad (I've experienced both). When you install a new one, remove the old because it could still drag down the voltage if it's bad.

Always, always solder your connections. It's a pain but worth the effort just for the reason you are experiencing now.

I tow a twin pack of SeaDoos with mine and it does great (but then I also tow the SeaDoos with an Eclipse and it does fine so maybe my set up isn't worth mentioning). I'll be towing a utility trailer with a Dingo in it tomorrow. If I run into any problems I'll post.

A lot will depend on what you are towing, not just the gross weight. Trailer length, width & load distrobution will all play important rolls in how it handles.