It's a wired system that I bought from this outfit: http://www.ebay.com/itm/7-REAR-VIEW-BACK-UP-CAMERA-SYSTEM-REVERSE-TRUCK-SAFETY-/260746847656?pt=LH_DefaultDomain_0&hash=item3cb5b925a8
I mounted my camera on the center divider inside the tank -- as high as I could get it and pointed at the feed roller. The IR LEDs in the camera illuminate the product so you can see inside the tank when the lid is sealed. When the tank is full you can see a ring of white blur. As it empties, focus improves so you can see the product and also the ladder rungs as a guide to how full the tank is. When the tank empties, you can see the roller turning and the individual flutes that expose themselves more and more as more product empties out.
I don't think a wireless camera would transmit very well from inside a metal tank but since any camera needs power anyways, the signal cable also has power wires and it's no extra work to run the combined power/signal wire. I routed the cable through the same place that the tank level sensor entered the tank. On my Fargo-Aire cart, that happened to be through the tank pressurization tube.