Hi Ben
You wrote:
planning to stop at the Khongor sand dunes for about 6 days.. I wondered if 6 days was too long... places to get food and water..."
Standard reply
Most travelers stay 1-2 nights at one of the few tourist camps alongside the sand duns (i.e Khongor camp 2Km from the sand dunes).
Dalanzadgad - about 180Km from the sand dunes, is a small town where you can stock up for food and water.
At the sand dunes you will only find the tourist camps and a few camel herders waiting for tourists.
The distances between the popular highlights in the Gobi Gurvan Saikhan National Park are large
Dalanzadgad - to Yolyn am (the ice canyon) 42km
Dalanzadgad - to Khongor sand dunes 180Km
The less popular reply
Let me take your question a step further.
Staying 6 days in one area brings a whole new aspect to traveling in Mongolia.
See a question above by Will titled "Travellers In Mongolia Who Are Not In A Hurry"
A lady woke up in a tourist van, looked out, and asked - what day is it? why do you ask? replied her husband, well she said, if it is Sunday so this must be a camel at the sand dunes, and if it is Monday so this is a dubble humped Yak at Huvsgol.
Mongolia calls for a slow travel paste, take your time, doing less in Mongolia is doing more.
Sit at the top of the sand dunes for several hours during sunset or sunrise for a few days and you will get spectacular sand dune shadows and colors most tourist miss.
If you come across a herder family, stay with them for a few days helping out with routine chores.
Links
Lucie Debelkova - non standard pictures of the sand dunes
Have a nice few days at any place in Mongolia or at the Khongor sand dunes also known as the Singing Dunes
Dan