Sorry for the lack of info, i appreciate its difficult to come up with solutions when its not clear exactly whats happening. I haven't been to see the exact generators in question but i'll try to give you as much as i know.
The generators are to be hired out for use on sites such as music festivals. They are large diesel generators.
There is a problem with them running out of fuel because they can be situated up to 1-2 miles apart. Obviously when they run out the vendors like hot dog and burger stands etc, get very upset
Because they are rented out it means i cnat start chopping into its wiring or anything hence the need to come with something which wont interfere with it.
I'm not sure where the fuel inlet is im afraid.
The measurement does need to be electrical due the requirement to send the information wirelessly back to a hub to monitor it.
I have considered the filler cap as an option. It would be quite easy to come up with something which could replace the filler cap, it would however need to be an exact fit.
I'm not sure if the filler is directly on the top, if it has a spout protruding from the top and whether it is straight or at an angle. Sorry again, i know this doesn't help much.
Thanks again for the help