If you wish to go by road anywhere off the RN7, you'll need either a 4WD or, if there happens to be public transport, a very strong stomach. Taxis-brousses can be brutal in the south.
At the time of research, the state of some major routes was as follows (many of these routes are impassable in rainy season):
RN7 to Tuléar Excellent sealed surface, but showing signs of deterioration; no 4WD necessary.
Tuléar to Ifaty/Mangily Excellent sealed road making the latter a viable base for the south.
Ifaty to Andavadoaka via coast road Sand track with rutted deep sand in spots, requires 4WD.
Ifaty to Andavadoaka via RN9 Usually quicker but potholed in patches; 4WD needed for second half.
Tuléar to Anakao via Betioky Terrible road but better closer to Anakao; security might be an issue; 4WD required.
Itampolo to Ampanihy via Androka Very bad, often rocky road, confusing tracks, maps not accurate. Use a local guide.
Ampanihy to Ambovombe Improved dirt road, particularly good on first half, but 4WD still needed.
Ambovombe to Ihosy Terrible road rutted by camions-brousses (large trucks) and not safe at time of writing, especially after dark.
Ambovombe to Fort Dauphin Terrible road, a deteriorated sealed surface with craters worthy of the moon; 4WD required.