In this paper we propose a monotonicity-based approach for decentralized voltage control of dominantly resistive DC microgrids with ZIP loads. For this purpose, we introduce the notion of set of assignable robust controlled decentralized invariants for the system. Then, upon selection of a desired invariant, an inner decentralized voltage control is designed and a criteria for convergence of the system's trajectories to an equilibrium point-in presence of constant power loads (CPLs)-is established. Interestingly, a simple realization of the proposed controller corresponds to a piece-wise voltage droop control, the gain of which are determined by the control specifications. A discussion on the selection of appropriate invariants is also carried out and the obtained theoretical results are validated on a 8-terminal benchmark.