Yes and no. I talked to Primetime about a u-shaped dinette and they protoyped one for us. But, they could not move the awning rearward like we requested and its placement was on our "must not"' list, so we moved on.
I've been told that the awning's location on my model was swayed by my input (there was internal debate and we were the proverbial straw that broke the camel's back).
I've talked to reps about minor changes to furniture in bunkhouses. And cabinets in outdoor kitchens.
I also have gotten a big fat "can't do it" from Brookstone regarding their 395RL for changes that I'd like to see.
Most of my requests were tweaks, not big changes.
What are you after?
I know there are threads and pictures where people took on their own camper's. One that I recall- a guy built bunks where his sofa lives. Another built a beautiful custom sofa.