This may be a contentious topic.
If you've got a month to take off work and a few extra thousand dollars laying around, get a CDL.
(WA requires 160 hours in classroom training for a class A. No option avoid, unless your employer has an approved training program, you're just getting out of the military, or have a CDL already.)
Dig through the PNW thread, someone has had a lawyer contact WSP about this and gotten an answer. I've called WSP and gotten the same answer. That answer has been no CDL is needed if it's a private vehicle you're using on your own. Call yourself, the motor carrier guys.
Be sure to be honest with your insurance about having a CDL.
Now, having said all that. You do need to know how to drive something that size if you haven't before. It's not something you can just start up and tool around in without maintenance and a constant vigilance for safety. There will be no small accidents. I have no idea if you know how to drive a vehicle that was 5-6x what the average car does. Empty.
Someone's insurance will go after you no matter what. They or their lawyer may smell blood because you've got a big green toy and that must mean you're loaded. I'm not going to predict how liability winds up for some hypothetical accident, beyond saying the most important part, to me, would be making sure my insurance had my back through the process.
There will be two parts to the accident followup, civil with the other party, and criminal with the state. Having required licenses et al. helps avoid the 2nd.