Is this a multiband antenna? If so, I'd worry about the wire coupling
different parts of the antenna to each other differently and requiring
retuning at best, making it impossible to get a match at worst.

If it's a single band vertical, or at least just a single pipe without
traps then inside is the best bet. What you probably want to do is put
an RF choke both at the top and bottom.

Actually, I would suggest NOT putting the sensor at the top of a
vertical, but rather put it a few feet from the top. The tip of the
antenna is a high voltage point, and the top choke will be much more
effective if it's at a lower voltage point where some current is
flowing.

Run the wires up inside, use twisted pair, put a hole in the side of
the antenna at a few feet from the top and attach an insulating rod
sticking out sideways from that point. Put a good choke on the wires
where they exit the antenna element both at the hole near the top and
at the bottom of the element.

If it's a trapped vertical or something you'll have to worry about the
extra coupling between antenna segments due to the wire.

If you're only running 10 watts (I think you said), I doubt you'll have
to worry too much about arcing and so forth in the trapped situation,
but it will probably detune the antenna quite a bit even if you run it
inside.

73,
Dan
N3OX