Friend of ours bought a 2012 Holiday Rambler Endeavor (sister coach to your Diplomat). This was the first year with Navistar motor. He has had some similar issues with DPF - he does not drive as many miles but when the DPF was determined to be cracked and got replaced under warranty, coach seems to be working fine now. Navistar was the only MFR to not use DEF for NOx control which meant they had to use lots of EGR. Did not work out well for driveability and so he has had several reflashes of computer for improved driveability.
