Catalyzes the 2'-O methylation of guanosine at position 18 in tRNA. Type I methylase, which methylates all tRNAs. guanosine(18) in tRNA + S-adenosyl-L-methionine = 2'-O-methylguanosine(18) in tRNA + H(+) + S-adenosyl-L-homocysteine Homodimer. Both N- and C-terminal regions function in tRNA binding, but the substrate tRNA is determined by the catalytic domain. Binding of TrmH to tRNA is composed of at least three steps: the first is bi-molecular binding and the subsequent two are uni-molecular induced-fit processes. Belongs to the class IV-like SAM-binding methyltransferase superfamily. RNA methyltransferase TrmH family.