The wood decking has rotted from the water leak, hence the cracking sound. On a Newmar, the CORRECT fix is to remove the existing rubber (probably Brite Tek). You do NOT overlay the existing deck on a Newmar. Newmar screws the wood subdeck on and you remove the existing wood and install new decking with screws. You have to replace ALL the decking if you are replacing the roof rubber membrane. Once you pull the rubber, new rubber will not adhere correctly to old decking that has the remnants of glue. Yes, the cost to do the job correctly is about $4000 to $6000 dollars if you pay someone to do the job. Check with your Insurance carrier to see if they will pay for it if you can show roof rubber damage. If lack of maintenance or poor reseal or repair of a leak, they probably will not the roof repair but will pay for the inside ceiling panel damage. The interior fabric roof panels will need to be replaced. The labor on a Newmar is 4.0 hours per 4 x 8 fabric panel plus the cost of the fabric. You remove the fabric and glue new fabric to the existing interior wood sub panel. Doug