hi,
I think 18-19 days for that route theoretically might be possible but it will be rushed.
There is no reason to visit Okavango or Ethosha if you just spend a night or so in the park.
I spend 3-4 days in Etosha, which was perfect, but certainly not too much.
Also, there are several areas/parks on that route which you miss: Chobe in Botswana, Waterberg Plateau in Namibia. Do you mean Sossusvlei by 'dunes'.
Some more questions: do you intend to drive all the time, or only in Namibia?
Also: from Okavango, are you going into Namibia through the Caprivi strip or directly west towards the Gobabis/Buitenpos borderpost, which connects to Windhoek?
I would skip Ethosha and the Skeletoncoast, as it's the longest detour. Instead I would concentrate on Chobe and Okavango, cross into namibia and visit the Sossuvlei/namib naukluft National Park, south to Fish River Canyon and on to Cape Town.
for example: 12-14 dec: vic falls
14-16 dec: chob
16-19 dec okavango
19-22 dec: en route to Sossusvlei
22-25 Sossusvlei and on to Fish River Canyon
26 and 27: Fish River Canyon
28-29: drive to Cape Town.
This leaves some days open, which is always better than fitting in too much.
Cheers
Michael.