According to the rules of accent, a "flat" verb (平板式) keeps flat in its て-form, and a non-"flat" (起伏式) verb is accented in the antepenultimate position in the て-form:
According to my books[*], ている-form keeps the accent location, so
The suffix -ます is accented as マ↓ス no matter the main verb is flat or non-flat, its naturally to deduce:
but "タ↓ベテイル" is not a regular accent for simple verb[**], so I am not sure:
タ↑ベテイマ↓ス (treated like a simple verb)
タ↓ベテ・イ↑マ↓ス (treated separately)
タ↓ベテイマス (with the accent of -ます neglected)
Which is(/are) correct?
[*] 三省堂 明解日本語アクセント辞典, and NHK 日本語発音アクセント辞典; neither talks about the case of "て+auxilary verb"; [**] Regularly verbs are either "flat" or accented penultimately.