Last year we found cracks on both frame rails under the axles on our 5er.  Took it to a trailer repair facility which repairs large commercial trailer frames.  I had previously contacted Lippert about the cracks, and they sent me repair instructions.  When I showed the owner of the trailer repair facility the cracks, he recommended repairing it virtually the same as Lippert.  So I felt reasonably comfortable in what they were going to do.  They repaired it for a reasonable cost and in only a couple of days.  Apparently, cracks in trailer frames are not totally uncommon, so I thought it was best to go with a repair shop that did such work day in and day out.