How are consecutive neutral tones pronounced? What are the pitch levels?
Zhang and Yang (2007:97) has an interesting remark on consecutive neutral tones within certain three-character words (三字組詞):

輕聲若同時出現在三字組詞的後兩個音節上，第二個音節的輕聲調型要根據第一音節的聲調變化，如同二字組的輕聲音節；但第三個輕聲音節一律都讀得短低，調型向下，類似模糊的去聲。如：
(Translation) Should both of the latter two syllables within a three-character word possess the neutral tone, then the pitch of the neutral tone in the second syllable should change according to the first syllable, much like the case of two-character words; but the pitch of the neutral tone in the third syllable should be read shortly and lowly without exception, with a trend going downwards, like a blurred fourth tone. E.g.:
拉下來 lā˥˥ xia꜋ lai꜌    了不得 liǎo˨˩ bu꜉ de꜊
剩下的 shèng˥˩ xia꜌ de꜌    爐子裏 lú˧˥ zi꜊ li꜌
坐下了 zuò˥˩ xia꜌ le꜌    跑過來 pǎo˨˩ guo꜉ lai꜊

'The downward trend' to me refers to the pitch change over the consecutive neutral tones (as a whole). In particular, the difference in pitch between the two neutral tones is most obvious when the first non-neutral syllable is in the first, second, or third tone.

Does the downward trend apply to two-character word groups suffixed with a particle too? There is no mentioning in the textbook, but I think mostly yes, albeit with the following observations:

As an assertive statement, there is only the inherent downward trend: 忙着呢 máng˧˥ zhe꜊ ne꜌, 算了吧 suàn˥˩ le꜌ ba꜌.

Some of the particles mark questions (e.g. 嗎, 呢), so it is important to take into account the effect of final rising on the pitch of neutral tones. Interestingly, for normal characters, there is also a difference in pitch between an assertive statement and a question:






Statement
Question




坐下了。 zuò˥˩ xia꜌ le꜌
坐下了？ zuò˥˧ xia꜊ le꜊


我的呢。 wǒ˨˩ de꜉ ne꜊
我的呢？ wǒ˨˩ de꜉ ne꜈





The final rising is best observed when the first syllable is in the 'low' half-third tone (i.e. it exceeds the effect of the downward trend): 怎麼了 zěn˨˩ me꜉ le꜈, 好了嗎 hǎo˨˩ le꜉ ma꜈.

The final rising may be attenuated when the first syllable is in the first, second, or fourth tone - they have a comparatively higher pitch than the half-third tone and stress is very easily placed on them, out of both semantical and phonological need (他 and 看 in the following examples). The net effect is that the pitch of the neutral tones now becomes roughly the same:






Statement
Question




他的呢。 tā˥˥ de꜋ ne꜌
他的呢？ tā˥˥ de꜊ ne꜊


看了嘛。 kàn˥˩ le꜌ ma꜌
看了嗎？ kàn˥˧ le꜊ ma꜊





Reference: Zhang Bennan &amp; Yang Ruowei (2007). [普通話連讀音變]. Hong Kong: Commercial Press.