It’s not clear if “mobile” in the question refers to vehicle fuel tanks, or vessels used for transportation of hazardous materials. 

1. If it’s a Type 4 vessel for transportation of hazardous materials, it will be built to a special permit that details the inspection requirements. Special permits vary but they are generally consistent on a 5-year requalification period, which also includes a visual inspection by an inspector certified to CGA C-6.2 at that interval. 
2. Since C-6.4 is mentioned, the question might pertain to vehicle fuel tanks. The C-6.4 document  recommends 36 months or less, but there are inconsistent requirements. The tank standard (ANSI/CSA HGV 2) originally required for a visual inspection every 36,000 miles or 3 years, whichever comes first, but the latest version states that the inspection is to be specified by the container manufacturer. There is an annex in HGV 2 that calls CGA C-6.4 or ISO 19078 to be used, although the ISO standard does not yet include hydrogen in its scope and is for CNG tanks only. Commercial vehicles such as trucks, buses and trailers also required an annual U.S. Department of Transportation (DOT) inspection to ensure that it complies with the Federal Motor Carrier Safety Regulations. This is a comprehensive check but does not have specific requirements for vehicle fuel tank inspection.