We always set up an RO system for the bigger customers with a brute garbage can with a float switch & heater so there was water ready for salt when we arrived. For the ones without it was a 42 gallon brute can, pump & hose.
A large pump capable of flow with head pressure, think sump pump & hose of different lengths. A few boxes of salt & whatever else you use regularly @ customers. Plenty of towels, an extra test kit if you need it, scraper blades, basic tools.
Try to standardize customers equipment, skimmers & pumps toward one mfg & carry a few pumps for them.