So this is basically a semi tractor with a 5th wheel to tow your 5th wheel camper? Not a "toterhome" as in it can be titled as an RV by itself?
I insured my GMC topkick cab/chassis with State Farm, but Ohio did not consider it commercial since it has no box and no commercial fifth wheel at this point. After the toterhome/RV conversion (with gooseneck or rv style fifth wheel) it will be retitled as an RV and State Farm will write a new policy as an RV. Ran all that by my agent before I started.
I would check the escapees forum, they have a lot of guys in your situation who have built RV toters, and have been through your issues. I would read up over there. I remember reading through a bunch of threads there about insurance and licensing issues, as well as d.o.t. issues. Unless you get that thing titled or licensed as an RV or non-commercial you are going to have to have cdl, logbook, etc unless you are hooked to your camper. I know every state is different on the titling issues, you might start at your local dmv, try to find a small office that has some time to talk about your specific situation.
Also, you may get it out of commercial designation if you junk the big commercial fifth wheel in favor of a RV hitch. That makes a difference in some states. Or possibly you may be able to install the minimum requirements to title as an rv in your state, which should make it easier to insure.
Try the escapees forum.