We have measured permittivity ε3, elastic compliance sE66 and piezoelectric coefficient d36 of a single crystal of RbH2PO4 in a wide temperature region above and below the transition temperature Ttr. All these quantities are shown to be anomalously large within the interval of about 35 K below Ttr, due to the presence of domain walls. In particular, the value of d36 exceeds 3000 pC/N in this interval. The model of pinned domain walls oscillating around their equilibrium positions predicts the relation Δε3σ = Δd36/ΔsE66 for the extrinsic contributions which is well satisfied by the obtained data.