S.R.
Hi A.,
My 5 month old girl is exactly the same way. It has surprised me because when my son was a baby, he slept good at night and took two good naps (though I think his naps were pretty short before he started to crawl). He is now 2 1/2 years and is currently sleeping more hours than his baby sister (12 hours at night + a 3 hour afternoon nap).
My pediatrician said that he's noticed that babies who sleep good at night early on tend to be slow about napping regularly.
Also, you mention that she fights sleeping during the day. So does mine. I think part of it is that "wow, the world is so amazing that I don't want to sleep or eat or do anything that might make me miss any of it" attitude that 5 month olds can get. My baby has begun to stop eating to look at things or listen to a noises, etc-- easily distracted. It seems to me that is the same sort of attitude that makes her not want to nap.
Anyway, I think that it will work itself out over time. It sounds as if you are doing the right things: mainly, putting her down at a routine time, in a routine way every day. If she needs more sleep, she'll start to take it. And maybe she just doesn't need as much sleep as other babies (or in my case, as her big brother). . .
good luck