Where does my 100w go? is it lost as heat in the tuner?
(my guess) Is it lost in the feedline due to high SWR at
the apex?


No; the tuner has fairly good QF components: there is a reason your tuner has
no huge fans or heat sinks.

Some lost as heat in coax from mismatch insertion loss. Much lost from ground
losses; some power now goes straight up rather than at lower elevations.

73,
Chip N1IR