Its also easier to modify the cab and chassis model vs a pickup bed rig which was sold complete. My Navistar flatbed dump does NOT have one of the gvwr badges in the drivers door that folks like to claim is the max a rig can go down the road at. It is up to the final bed manufacture to certify the gvwr etc of the rig. Hence, why many class C rv's have tag axels, all after market to increase gvwr. Same with drop axels on dump trucks etc. A legal modification to increase GVWR! One can change out tire/rim sizing, spring pack etc to legally increase gvwr.
In some circumstances, yes it can be a better option than a pickup with a bed on it from the chassis manufacture.
BUT as you note, most will be vinyl or cloth seats, vinyl floor mats, a basic ac system and radio if at all! If you want a really nice interior, you need to look at rigs with RV beds, then most of those were purchased with the leather pkg from the came from the chassis manufacture many times. OR it could have been aftermarket added and changed out.
Then again, some of us prefer vinyl floors even if we have leather seats per say, as you can get in with 3" of snow or mud on ones boots and not destroy the carpet. Or no leather like myself, prefer cloth over both leather or vinyl. BOTH of these are too warm on a sunny day, and really cold in winter months. Cloth is much nicer....My .02 thank you very much!
Marty