  We prepare and analyze Rydberg states with orbital quantum numbers $\ell \le
6$ using three-optical-photon electromagnetically-induced transparency (EIT)
and radio-frequency (RF) dressing, and employ the high-$\ell$ states in
electric-field sensing. Rubidium-85 atoms in a room-temperature vapor cell are
first promoted into the $25F_{5/2}$ state via Rydberg-EIT with three infrared
laser beams. Two RF dressing fields then (near-)resonantly couple $25 \ell$
Rydberg states with high $\ell$. The dependence of the RF-dressed Rydberg-state
level structure on RF powers, RF and laser frequencies is characterized using
EIT. Furthermore, we discuss the principles of DC-electric-field sensing using
high-$\ell$ Rydberg states, and experimentally demonstrate the method using
test electric fields of $\lesssim$~50~V/m induced via photo-illumination of the
vapor-cell wall. We measure the highly nonlinear dependence of the
DC-electric-field strength on the power of the photo-illumination laser.
Numerical calculations, which reproduce our experimental observations well,
elucidate the underlying physics. Our study is relevant to high-precision
spectroscopy of high-$\ell$ Rydberg states, Rydberg-atom-based electric-field
sensing, and plasma electric-field diagnostics.
