Consumer presents a state driver's license with a street address that is postal mail box service (PMB1234 is listed) frequently used by RV dwellers. As allowed by CIP, we obtained a street address of a next of kin.

CTR instructions say to use a "non-location address" only if a street address is not available.

If we need to file a CTR, do we use the customer's mail service street and PMB address or the next of kin's street address?

You use the physical street address of the individual's residence. Do not use the private mailbox service's address (the dude doesn't live there). Unlike CIP rules, a business address or a relative's address isn't acceptable. If a street address is not available for the individual's residence, you can use a non-location address.

He lives in a RV, stays in a location for few weeks or month then moves on. He does not have a physical address.

Well, he has an address at the time of the transaction, unless he's in your drive-thru on the way from one place to another when the CTR is needed.

...the local KOA Campground, Walmart/Sam's Club parking lot, State Park or our drive through don't seem to be very useful. The next of kin is more likely to be helpful to the authorities if a CTR/SAR situation needs to be investigated.

So, I feel like I'm back to my starting point.

John answered your question accurately; e.g. the "next of kin" thing from CIP isn't relevant to the CTR.

Whether the individual lived in a stone house for 30 years or a walk up apartment for 30 days, that location is still his address. However, either could change tomorrow. If the person is an RVer and today's address is not guaranteed to be tomorrow's address there's no difference.