Bryan's answer about stretching a Chevy must have bearing on why those owners of stretched Chevy chassis aren't treated as well with driveshaft problems as the Ford owns are...
I think it's provable that the Ford chassis lasts longer than the Chevy. But ultra-high-mileage Class C's are very rare. A Chevy will most likely serve well for the life of the rest of the RV.
GM hasn't shown the commitment to the RV chassis market that Ford has, at least over the last couple decades. Yes, there've been a lot of Class A's on P30's but the F53 caught up BEFORE GM eliminated the 8.1 gas V8. Of late, a gas Class A has to be on F53 and a diesel "puller" will be a Workhorse chassis with a diesel like Navistar. Then in C's they offered a too-small 5.7 V8, then a good 7.4 V8, discontinued it, tinkered with the GVWR numbers, offered the good 8.1 V8, dropped that. Then the 4500 and are they really available to build on...?
All through this Ford offered chassis, higher GVWR, powerful 7.5 V8 then the excellent 6.8 V10. And a little later they sealed the chassis deal with TorqShift.
GM may have a smaller service network than Ford but at least the 4500 is made up of parts that are going to be available. Maybe you can't get a "4500" badge and a new door or fender'll say "3500" or nothing, but you can always keep a Chevy running.
So if you can find a Chevy, you want it, buy it. You're a consumer and only buying ONE for your personal long term use.
But move to the other side of the table. You're an RV builder. Do you want to plan X-Thousand units not knowing what the rating and availability will be? OK, you tool up to build on Chevy. What about next year?
Yes, Ford is ending the E-Series cutaway line. But they're being gentleman enough to say WHEN so builders can plan.