Catalysis of the reaction: dTDP-6-deoxy-beta-L-mannose + N-acetyl-alpha-D-glucosaminyl-diphospho-trans,octacis-decaprenol = dTDP(3-) + alpha-L-Rhap-(1->3)-alpha-D-GlcpNAc-1-diphospho-trans,octacis-decaprenol + H+.
decaprenyl-N-acetyl-alpha-D-glucosaminyl-pyrophosphate:dTDP-alpha-L-rhamnose rhamnosyltransferase activity