If you had a high electrical leakage situation with a device plugged in to the trailer, and if you had poor grounding between the trailer chassis "ground" and true earth ground, then that would explain the AC current on the skin of the trailer.
The evidence pointing to the power converter being the high leakage device, and the probable assumption that it has an internal switched mode power supply makes me think of a seminal incident in my career as a Hospital Biomedical Electronics Technician. Going back to the 1970's, much of the growth of hospital biomedical engineering departments can be attributed to a magazine article by Ralph Nader (based on a single faked/flawed study by a now disgraced/exiled biomedical researcher) that asserted that 10,000 patients a year were dying in hospitals from microshock. Microshock is a shock inadvertently applied to the patient through the low resistance monitoring leads and catheters. Within a few years while this "study" was being debunked, National Medical Equipment Safety organizations were being set up, limits established, and all patient care equipments gotten onto safety inspections both while new, and on a regularly scheduled basis. The electrical leakage limits with the ground wire open for medical equipment were set at 100 millionths of an ampere for general usage, and 10 millionths of an ampere for electrode patient connected devices. The larger spec is about 1/10th of the level people can feel, and about 1/100th of dangerous current levels. Again, these specs are a worst case scenario, with the grounding wire open (broken). Proper grounding is also checked, along with the amount of current patient leads will sink - testing if the patient touched a live circuit how much maximum could be grounded by the patient leads. I wrote this paragraph so you would understand my technical story following, and so that you and your loved ones would feel more at ease when connected to patient care equipments.
In the 1980's the Personal Computer came to the forefront, and we began to see medical devices based around PC's, instead of custom designed electronic boxes for every needed medical device. The first IBM PC based device came into our shop, and I was assigned to do the incoming inspection. Well, the IBM PC FAILED the electrical leakage test. It seemed that ALL PC based systems were failing the stringent hospital electrical leakage tests. I got a schematic of the PC power supply (where the AC is), did an analysis of the primary AC circuitry, and found that the standard AC power line filter used on all PC's had capacitors from the live and neutral wires to ground. These are meant to ground out high frequency transients from coming in/going out to the AC power line, but pass much more 60 cycle AC current than the medical leakage restrictions allow. I clipped out these capacitors and the PC passed Hospital electrical safety, but now was questionable if it met FCC radiated electrical noise limits. Very soon there were available Hospital Grade power line filters which met the industry leakage standards.
So, what I am getting at is that with a Switched Mode Power Supply converter, the power line filtering capacitors may be conducting more 60 cycle AC than intended to ground, and with a poor system ground to true earth ground, be setting up a hot skin shock situation, while still operating as a converter/charger normally.
