Scholars have long debated what Paul's tentmaking actually entailed, as we don't have any detailed descriptions of this line of work in ancient literary sources. Moreover, while "tentmakers" seems to be the most straightforward interpretation of the intended meaning of σκηνοποιοί at Acts 18:3, the term itself can also imply a range of different activities, from tanning to the building of theater sets.

One of my former Greek professors likes to quip that Paul was a defense contractor, assuming that his customers would have included the Roman army. Conceivably, the legions stationed around the Empire may have turned to private artisans to supplement or replace their tents, which otherwise appear to have been produced by imperial slaves and freedmen. However, we also have plenty of evidence for the use of tents by pilgrims, and I think it's more likely that they and other traveling civilians would have represented the main clientele for the average urban-based tentmaker.

The socio-economic status of tentmakers remains unclear. On the one hand, tentmaking definitely fell into the category of manual labor and may have involved particularly unpleasant work (e.g., tanning leather), but on the other, tentmakers' artisanal skills probably placed them above common laborers in the eyes of the populace. And although artisans' workshops were generally family-run enterprises, they frequently employed slaves and could become quite large and lucrative, with the owners acting in a managerial capacity rather than engaging in hands-on work (at Acts 20:34, of course, Paul emphasizes that he has indeed worked with his own hands).

If you're interested in reading further, Holger Szesnat's article "What Did the ΣΚΗΝΟΠΟΙΟΣ Paul Produce?" (Neotestamentica 27 [1993]: 391-402) discusses much of the relevant literary evidence as well as the earlier scholarly literature.