X-ray and neutron diffraction techniques are combined with first-principles-based simulations to derive and understand the structural properties of Pb(Sc,Nb,Ti)O-3 (PSN-PT) near its morphotropic phase boundary (MPB). An analysis of our measurements yields, at room and low temperatures, an overall tetragonal T-monoclinic M-C-monoclinic M-B-rhombohedral R path (when adopting the notations of Vanderbilt and Cohen, Phys. Rev. B 63, 94108 (2001) for the monoclinic phases) as the Ti composition decreases across the MPB. A composition- and temperature-dependent significant mixing between some of these phases is also measured and reported here. The overall T-M-C-M-B-R path, which has also been proposed for Pb(Mg,Nb,Ti)O-3 [A. K. Singh and D. Pandey, Phys. Rev. B 67, 64102 (2003)] is rather complex since it involves a change in the polarization path: this polarization first rotates in a (100) plane for the T-M-C part of the path and then in a (1-10) plane for the M-B-R part of the path. Moreover, a comparison between these measurements and first-principles-based calculations raises the possibility that this complex path, and the associated M-C and M-B phases, can only occur if the samples exhibit a deviation from a perfectly homogeneous and disordered situation, e.g. possess nanoscale chemically-ordered regions. If not, homogeneously disordered PSN-PT is predicted to exhibit at low temperature the same polarization path as Pb(Zr,Ti)O-3, that is T-monoclinic M-A-R which involves a "single" polarization rotation in a (1-10) plane. Nanoscale inhomogeneity may thus play a key role on the macroscopic properties of PSN-PT, in particular, and of other heterovalent complex solid solutions, in general, near their MPB