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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 the $25F, 25H(\ell =5)$, and $25I(\ell =6)$Rydberg states. 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 paper is relevant to high-precision spectroscopy of high- $\ell$Rydberg states, Rydberg-atom-based electric-field sensing, and plasma electric-field diagnostics. Published by the American Physical Society2024