Is a non-transportation related mobile fuel tanker with more than 55 gallons locationed near a 55 gallon drum storage area considered a facility?

Perhaps.  "Facility" is defined in Part 112 to mean "any mobile or fixed, onshore or offshore building, structure, installation, equipment, pipe, or pipeline (other than a vessel or public vessel) used in oil well drilling operations, oil production, oil refining, oil storage, oil gathering, oil processing, oil transfer, oil distribution, and waste treatment, or in which oil is used, as described by Appendix A to this part.  The boundaries of a facility depend on several site-specific factors, including, but not limited to, the ownership or operation of building, structures, and equipment on the same site and the types of activity at the site" (§112.2).  An SPCC Plan applies to a facility, not to individual oil containing structures; there is no distinction between who owns and operates the oil containing structures at the facility.  If it is unclear as to whether or not the fuel supplier's equipment is considered part of a specific facility because it is on your property, EPA Regions can provide a site-specific interpretation or guidance regarding the facility.  You should workout an agreement with the equipment owner as to whom, if anyone, must provide an SPCC Plan.