What was this vehicle doing?
This looks like an Aircraft Fuel Servicing Cart, which transfers fuel from an underground network to the aircraft. There are two types of vehicles that transfer fuel at larger airports with these fuel networks:


Hydrant trucks:


(image source: Wikimedia)


  Aircraft fuel servicing hydrant vehicles do not have tanks. These vehicles
  connect to a pressurized Airport Fueling System hydrant and transfer the
  fuel to the aircraft through a filter.


(ICAO - International and FAA Fuel Fire Safety)
Towed Carts:


(image source: aeroexpo.online)


  Aircraft fuel servicing carts are equipped with facilities to transfer fuel between a
  Airport Fueling System hydrant and an aircraft and are normally parked at a fixed
  location near a gate.


(ICAO - International and FAA Fuel Fire Safety)


The cart in your picture seems to be the latter of the two (a different model though). You can see the hose that will be connected to the aircraft on the metal staircase. The other hose on the ground connects to the hydrant (hidden behind the aircraft in your image). Your picture shows an Airbus A321, which has the fuel coupling typically located under the right wing (number 11 in the following image):


(image source: Airbus A321 Aircraft Characteristics and Airport and Maintenance Planning Document)


  11 − REFUEL/DEFUEL COUPLINGS (OPTIONAL−LH WING)


Your picture was taken at Charlotte (CLT), which according to this page is equipped with a Jet A Fuel Hydrant Facility. Airports without an underground fuel system will use tank trucks (self-contained fuel trucks) instead:


(image source: shell.com)