I think a great deal is made of "frame failures" that isn't necessarily warranted. It is a good idea to inspect the frame at least annually or any time handling or tire wear would lead you to suspect a change in alignments and rigidity, but, I wouldn't worry a great deal about the frame. Lippert, unjustly IMHO, takes a lot of hits on their frames. When you make 98% of the frames on the road, 98% of the failures will be yours...just simple math and all else being equal. The frames, and this is the biggest thing, are built to the ordering company's specifications. So, yes, you may find that a particular brand's frames are failing more than normal, but, that would lead me to believe that the frames were under specified; Lippert has no idea what the final deployment of their frames are intended for.
As for toy haulers being more susceptible? I suspect they are, and they are susceptible to a whole host of problems not encountered with regular bumper pull trailers; light tongue weights, excessive loading, inadequate frame strength for the drastically unbalanced loads that people put in them, poor handling, insufficient strength of the suspension, etc. They are a rather new direction in travel trailer design and it will take some time to get the kinks ironed out, but, IMHO, a lot of the problems are owner induced. Because the rear end of these things are so cavernous, people load them up...not one ATV, but, 2,3 and I saw one with 4 in it, not just a single motorcycle, but, multiple motorcycles of the heaviest touring type and then since there is space left over, lets put in a big generator, maybe a week's worth of wood or more, cases of your favorite beverage, a big household style grill with several spare propane tanks, a small refrigerator so that the "cold ones" are more accessible, etc. All of this weight is in addition to the regular load that folks put in it for personal use; clothes, food, pots and pans, etc. However, most of the heavy stuff winds up in the "garage" and that is the worst place for it...concentrated in one area and
behind the trailer's axles. This leads to unequal dynamic forces on the frame, unstable light tongue weights and unequal load carrying of the axles and suspension components. Realize the extra flexing that goes on with that weight behind the axles...the most unstable area on any trailer since it is only supported on one side (the axle side). This leads to a great deal more flexing of the frame of the frame at the axle location (like a hinge point).
So, yes, there are inherent issues with toy haulers that you won't find with regular travel trailers and there are "growing pain" issues that a lot of manufacturers will encounter with trying to develop adequate strength for their units and still maintain a profit margin. If you have a toy hauler, keep the above in mind when loading, check your tongue weights (some even have to carry a full fresh water tank to keep the trailer tongue heavy with the load distributed so that the suspension is loaded per design specifications. Also bear in mind the frames are delivered as specified by the ordering manufacturer and what one company specifies is not necessarily what others specify and "problems" are probably not "cross manufacturer" applicable. FunFinder has its quality control issues, as does every manufacturer, but, in my experience, frame strength and reliability hasn't been an issue on any of their products to date, but, that, too, can change. As always, at least annual inspection of all components (spring and fall is even better) and any time you suspect something may have happened (hit a curb recently? Check the suspension) and that inspection should include everything on the trailer from the ground to the roof. Load carefully (for any trailer, more important for toy haulers) and respect tongue weight, load location and the GVWR of the trailer.
Travel smart and travel safe!
