You dont have to store more water...
Just lose less water. Camels don't store water in their humps; that's just fat. They store water in an internal bladder.
Animals lose water either through general/unintended I'm gonna say osmosis but thats not quite right. With water being leeched from their skin from the dryer air. This happens most when we open our mouths.
They lose water because of perspiration. We sweat to cool ourselves down.
They also lose water because of secretion. We gotta get rid of waste somehow.
All your creature has to do is not need to sweat, capture lost water from when opening its mouth (a lot of hair maybe), and defecate dust.
Some animals evolve spines to capture moisture and draw it closer to their body for collection in desert environments.
Saharan snakes are cold blooded so they don't need to sweat in the desert. They also burrow into the sand at noon to avoid to much heat.
You could justify having a cold-blooded unicorn with reflective skin, a long snout it breathes in and out that is packed with hair and acts as a filter and water catch. It can also poo rainbow colored dust. Would be perfectly adapted for the desert.
Edit: Since there is a desire for a chemical approach to this:
Hydrocarbon chains!!
Hydrocarbon chains is practically synonymous with oil. They are composed of chains of carbon with hydrogen caping the ends. Your creature could just drink water and convert carbon and hydrogen into these. It could then perform a controlled combustion reaction to convert these into H20 and CO2 by taking in air. This is an incredibly dense way to store energy and water. It is also not completely unrealistic. Plants store energy in glucose which is essentially a hydrocarbon and some nitrates i think. When it undergoes respiration at night it indeed release this water which is what can often, in combination with other conditions, result in muggy or foggy conditions.