I bought a 1986 C3500 from a couple in 1991. It was registered as a non-commercial truck because they had a campershell on it. I took the shell off and installed a 5th wheel hitch. To this day I have never had a problem, it's still registered as non-commercial.
My 2007 C4500 is registered as a commercial truck the fees this year were $588. Because it is 15000lb gross and even though I only use it for towing my 5th wheel the DMV indicated it had to be commercial. Below 8600lbs seems to be the point where you can get away with non-commercial.