Water
Water is no problem on Misti. Unless you did not bring enough. There is none on the mountain.
Leave the filter in Arequipa. Carry at least 7 litres / person for the 3-day trek. At least 5 litres for the 2-day climb.

pack loaded with 7 litres of water
In Arequipa you can buy 2.25 litre water bottles inexpensively.
Unfortunately water weighs 1kg / litre. So much for saving weight by leaving the filter behind.
Campsite
We have only stayed at Monte Blanco (4800m) on the 3-day trek.
It is a classic base camp with a wonderful view over the nothingness of the Reserva Nacional Salinas y Aguada Blanca.
On our recommended 3-day route you stay at Monte Blanco 2 nights giving you a chance to settle in. A home away from home.
It can be cold & windy. Tie down & weight your tent well within the protective stone walls. Be sure a big gust cannot Maytag it down the valley while you are gone.
Cooking
All cooking must be done on a lightweight camping stove. We bring a back-up stove in case our primary stove breaks down.
Meals should be as simple & quick-cooking as possible. Instant mashed potatoes are a staple in South America. We rehydrate in soup rather than water.