Ford E350/450 or Chev 3500

Two reasons why a manufacturer might not use or offer the C3500:

1. A C3500 (or E-450) does not have the capacity to carry that model RV.

2. If small enough for a C3500, the manufacturer might have negotiated a better price on the Ford chassis.

Ford E-450 has the highest GVWR and allows the longest wheelbase stretch. The recent GM 4500 comes very close.
But you asked about GM 3500... Lower ratings, as with E-350, may affect your decision. I want to offer a consideration based on the little that I know about GM chassis vs. Ford - The Ford Motorhome chassis seems to always include their top transmission offering for the model year in question. In other words, when they offered TorqShift, that's what you got. It seemed to me that some of the GM models, even with 8.1 engine, got a Hydramatic-Something and not an Allison. There's nothing wrong with a GM Hydramatic. Of all the brands we've owned, we had less transmission problems with GM than Ford, even the legendary Chrysler "727." But if you want a transmission with all the tricky goodies, look carefully to be sure the chassis you select has those features.
My understanding may be dated, since it now seems GM is using a Six-Speed Hydramatic that looks like it has Allison-style controls.
Our Ford has the 4R100 that preceded the Allison-like TorqShift. It operates about like the plain Hydramatic would. Has "Overdrive Off" and NOT the "Tow-Haul" with modified shift strategy, engine braking and the like. Works fine, just not as much fun as if it had all the tricks.
If it would fit your capacity requirements, I'd say don't shrink from a 3500.

With our 28ft and the 3500 we wouldn't be able to tow that 2800 if you go by the numbers. Granted we may pull a car but very seldom and not over long distance. Some one told me once always buy one engine/drive train size bigger than you think you'd need.. of all the vehicles I've owned I've been happier when I stuck to that.

The problem with most RV's is that they do not have enough cargo capacity once the fresh water tank and fuel are full.

I would still pick the E-450 to get the additional cargo rating. It would mean if you upgrade to a 5,500 pound cargo trailer with a 600 - 750 pound hitch weight will be no problem.

I have a 97 Bounder with a 2,734 pound cargo rating, weight of the RV without options, water or fuel. I have checked it a couple of times while on a long trip, and have been very close to the GVWR. But that is me. I have several options, and even leveling jacks add about 300 pounds to the RV base weight.

Fred.

While I do own a Chevy 3500,so it is a biased report. I have had no problems with mine as many others. Here is a THREAD That will answer some if not all of your Questions about it.

If you can get thru it. It is fairly long. And you may have to get out your Fire Suit....:)

A bit of additional info: Just 2 people, might sometimes tow a small trailer with a couple of dirt bikes, but would also like to be able to tow a ski boat, about 2800 lbs including trailer, if I need to. We are used to a truck and camper, so we generally pack light.