Skid unit. Example of a 200gal skid unit mounted in the bed of a custom rescue vehicle. Skid unit mounted in an ATV. A Skid Unit (called a Slip-on in Australia) is the common name used to refer to a complete self-contained fire fighting apparatus designed for use on/in commercially available vehicle platforms. Vehicles such as pickup trucks, flat bed trucks, vans, off-road vehicles, trailers and others can easily be fitted with a proper size skid unit for a variety of fire fighting operations.