The problem is getting to TUN: it costs anywhere between 300-500 EUR if you book with LH, 267EUR with Ltur lastminute on LH.
Doing a turn FRA-TUN-FRA is perfectly possible:
Hin Frankfurt/Main (FRA) Tunis (TUN)
06.04.2008 um 22:25 Uhr
07.04.2008 um 00:40 Uhr 0 LH 4110
Rück Tunis (TUN) Frankfurt/Main (FRA)
09.04.2008 um 03:55 Uhr
09.04.2008 um 06:15 Uhr 0 LH 4111
But you basically are awake all night. Also consider the option of getting a second passport for the Tunisia entry/exit stamps, especially if you intend to enter the united states.