  We present X-ray through submillimeter observations of the classical RV Tauri
(RVb-type) variable U Mon, a post-AGB binary with a circumbinary disk (CBD).
Our SMA observations indicate a CBD diameter of $\lesssim$550 au. Our
XMM-Newton observations make U Mon the first RV Tauri variable detected in
X-rays. The X-ray emission is characteristic of a hot plasma ($\sim$10 MK),
with L$_{X}=5\times10^{30}{\rm erg}~{\rm s}^{-1}$, 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 $\sim$60 yr, possibly caused by a feature in the CBD.
From literature radial velocities we find that the binary companion is a
$\sim$2 M$_{\odot}$ A-type main-sequence star. The orientation of the binary's
orbit lies along our line of sight ($\omega = 95^\circ$), 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 ($\sim$4.5-9 au) to be comparable to the binary separation,
implying that one or both stars may interact with the CBD at apastron. The
obscuration of the post-AGB star implicates the companion as the likely source
of the enhanced H$\alpha$ observed at RVb minima and of the X-ray emission that
may arise from accreted material.
