Department of Transportation (DOT) portable propane cylinders (types 4B, 4BA, 4BW, and 4E), which are normally used in your travel trailer or fifth wheel, need to be recertified 12 years after the manufacture date. (Some other propane cylinders may have a 10 year recertification date) After the initial recertification, they will need to be recertified every 5 years, if requalified by visual inspection, which is how most RV DOT cylinders are recertified.1