Dave,
I have read on this forum now and again over the very same concerns you mention. It is common sense to me to avoid such a long rig. If the rear overhand was less, placing more weight forward where it could be better shared with the front axle, then I might feel differnetly, especially if on the later 2008-2014 chassis which have a heavier front axle rating. As others mentioned earlier, the rig would also handle better.
30+ feet and a significant rear overhang on an E450 cut-away chassis is a poor combination in regards to over-loading the rear axle & tires there. Increasing the risk further is that some people purchase those huge rigs to accomodate their family which adds even more weight. Then load up everyone's belongings, food, etc. and you are in danger. It is under those conditions where most rear tire blow-outs occur because even the duel rear tires are over-loaded.
Therefore, I cannot reccomend considering one of those rigs unless you are traveling very light, only two people carrying few things, no fresh or gray or black water, all in combination to keep it light. But to me that defeats the purpose of going "big" in the first place. I would either go with a Super-C or a class-A so that both the chassis and bigger/stronger tires would safely handle the weight.