What is a R-5 fuel truck?

The R5 refueler was first in production in 1966 and ran to 1968. It was a GMC, V6 gas engine. It was a cab over design. Prior to the R5, The R2 was the main unit used by the Air Force. Both were 5000 gallon tanks. The R5 had a low profile tank and the R2 had a large round tank. The R2 did not have a high level shut off valve so the driver had to be on top when it filled. The R2 had a Ford cabover on the later units (62-64)and a GMC on the early units.(58-??)