no cracked frames to see.... they were built properly and boxes put on them with this in mind. There is I think only four or five solid boxes on LMTV's out there right now... but only about 4 years?? at most of minimal use..... time will tell... Id expect damage not to show up till 10+ years.... maybe less with full time travel.
chassis is suppose to flex a little..... when it is not allowed to flex as designed it will crack over time. As you describe the M1079 ... this is a good example..... of solid built; non flexible big box. Has nothing to do with weight. Box has no movement in it and prevents whatever it is attached to from moving as well. ..... thus if bolted "solid" direct/indirect to truck chassis, it prevents truck chassis from flexing........ thus it will eventually crack something. Even possibly transmission case. Particularly since front half of truck is trying to flex a bit as designed; while rear half is being prevented from flexing. Hence why 1079 has captured springs. Type flex that is the issue is chassis twist.
A not so solid box... opposite happens..... the truck will do its thangg and flex..... making the box flex too.. causing whatever is solidly attached to the box walls to slowly rip apart.
if anyone is interested... I have brainstormed on how to go about it different. and mount interior parts in a way that allows camper box to twist w/out damaging them .... then build box that has some flex... enough to not limit truck so much... just different way to skin a cat. Not sure though how to build a box that can over its life time be happy twisting .... yet be able to insulate it well... much less not crack the box frame.
Don't think our frames flex this much..... but this video shows the point. He addresses off road being better allowing truck to twist.... but he does not address that it could overtime damage truck chassis by preventing flex truck is suppose to have.
PS-- if you want good reading on chassis and expedition truck stuff.... this has been argued to adnauseam on Expedition Portal