Your fears are true and valid. After all, what you are getting is much, much worse than any sort of prison sentence. Being a loincloth to Tarzan would be bad enough but to be the one in which he relieves himself is a million times more awful! In the movies, Tarzan is not seen going through all of these natural - though vile when you are on the receiving end - urges. He is just shown swinging from vines to vines and swimming and overall enjoying life in the jungle. None of the messy parts, never. And Tarzan just lives in a miserable hut in the branches of a high tree. Barely more than an ape's nest, in fact. He sleeps on a pile of large green leaves and he has a few bowls in a corner. There is no indication of a human presence, let alone of a woman. Jane, in fact. Looks like she is not around, or at least not yet, because no girl or woman would be willing to live here, that's for sure!
Anther sign is that Tarzan strokes you and relives himself into you at least once a day and it's absolutely vile! Seeing him reliving his body of its wastes is barely more tolerable. If Tarzan had Jane or another woman around, well, it would mean it'd be awkward to watch the jungle man have sex but that would also signify less cum dumped into you.
And Tarzan knows who you are! He knows you're a criminal serving a sentence and he is more than happy to remind you that HE is the instrument of justice. You can only wait and be unconscious for long stretches of time and curse silently at the unfairness of your situation. However, you do not reflect about how you deserve it fully, considering the heinous crime you've committed.
Then, one day, Tarzan removes you and washes you thoroughly in a stream, which feels very much like a reprieve. Tarzan then gets a new loincloth. Is he finally changing? Great! For you it will be the greatest reprieve. But as you are hung out to dry, Tarzan explains to you that he is meeting with a trader and that you're the goods to be exchanged. But how could that be? The jungle man explains to you that he will change you into something the trader will want very much. Tarzan is happy to get cans of fruits, clothes and metal objects in return, as well as other goods. He ten blows powder at you. You are happy to become alive again - assuming you won't be another object - but you are also very frightened at what you are about to become.