Lasse,
If you don't find a reliable video solution I would recommend that you look at using an Intrinsically safe Motorola portable radio such as is used by Fire personnel in HAZMAT environments. These radios won't blow the place and your people up because of explosive gases when they transmit. They automatically notify a local base station of a man-down event with and alert, as well as open the radios local microphone to monitor local audio, as well, as you can speak with the person carrying the radio to check his or her status. For safety reasons, It is typical that the person carrying the radio cannot clear the man-down alarm after it is triggered. The man-down alarm must be cleared by human intervention at the base station. The controlling base can be located locally. Keep in mind that regardless of the system you choose, metal tanks will not pass RF signal very well, if at all. So, you will probably need at the minimum a passive radiator antenna system or an RF repeater with antennas to make sure you have a 100% 2-way radio coverage between the inside and outside of the tank.
Your request sounds pretty simple on its face but when life and safety is at stake it gets more complicated pretty quickly.
Hope this helps