I don't have an answer for you, only another train of thought to consider. There are plenty of horror stories that involve Lippert frames that have failed in one way or the other.
However, on the other hand Lippert is and has been the largest mass produced RV manufacture for years. I feel sure that the number of failed frames vs the number of frames produced in a small percentage number. However, I do understand that this is little comfort to those who have experienced a problem.
Also stop to consider that not all frames that come off of the assembly line will be equal when it comes to material used or the welds applied.
IN the mean time I would simply enjoy your RV, if a problem does come up address it at that time. There really is no way to predict metal fatigue before it occurs. You can inspect the frame from time to time if it makes you feel better to try to say one step ahead of a major problem. But getting to this area of the frame is not the easiest job to do.
How was that for a long winded reply that really offered no answer to your direct question.
2018 Berkshire XL 40B
Past RVs: National Pacifica 40', Kenworth T300 Toterhome, Jayco Class C 32'. American Eagle 40',
American Eagle 40', Bounder 38' tag axle