I love disc brakes and wouldn't be without them, but I've never seen disc brakes on a semi-trailer and they have no issues locking those up when necessary.
I think the issue with drum brakes on an RV trailer/5th wheel is the actuation system as much as the drum brakes themselves. The magnetic system seems to be a pretty poor setup IMO. A good drum brake should have no problem locking up the wheel, but that seems to be a real issue with magnetic actuated trailer brakes. I've heard that increasing the wire size can help with the issue, but am not positive, in that I have not done it myself.
I've never heard if there is an add-on actuation system that would work decently. I guess if a person would go to that expense then disc brakes wouldn't be that much more. I think I would just as soon avoid the extra complexity of the hydraulic system, not to mention the huge expense, but I would like something that could take my brakes right up to the maximum.