We present an X-ray through submillimeter observations of the classical RV Tauri (RVb-type) variable U Mon, a post-asymptotic giant branch (AGB) binary with a circumbinary disk (CBD). Our SMA observations indicate a CBD diameter of <~550au. Our XMM-Newton observations make U Mon the first RV Tauri variable detected in X-rays. The X-ray emission is characteristic of a hot plasma (~10MK), with L_X_=5x10^30^erg/s, and we consider its possible origin from U Mon, its companion, and/or binary system interactions. Combining DASCH and AAVSO data, we extend the time-series photometric baseline back to the late 1880s and find evidence that U Mon has secular changes that appear to recur on a timescale of ~60yr, possibly caused by a feature in the CBD. From literature radial velocities we find that the binary companion is a ~2M_{sun}_ A-type main-sequence star. The orientation of the binary's orbit lies along our line of sight ({omega}=95{deg}), such that apastron corresponds to photometric RVb minima, consistent with the post-AGB star becoming obscured by the near side of the CBD. In addition, we find the size of the inner-CBD hole (~4.5-9au) to be comparable to the binary separation, implying that one or both stars may interact with the CBD at apastron. The obscuration of the post-AGB star implicates the companion as the likely source of the enhanced H{alpha} observed at RVb minima and of the X-ray emission that may arise from accreted material.