Take this for what it's worth, for it is just my opinion.
If I were going to invest in a 30+ foot rig with slide outs, I would surely look at a class-A over a C for I feel C's that long with slide outs are too close to, sometimes over their weight limit. Not just the chassis, but even the tires become over-loaded, running a higher risk for blow-outs.
A few years ago at a rest area, we parked next to a 32' diesel pusher with one massive full-wall slide out. The owners were kind enough to show us around, inside and outside. I thought that was such a sweet rig considering the short length for a DP. It had mobility & very comfortable full-timing written all over it.
I feel that is the right way to go if you can afford it. If we were to go with a 30 foot rig, I personally would add the 2 feet and look at used DPs to work with our finances.
HERE IS A 2002 Beaver Monterey 30" diesel pusher, so 30 footers exist.