Hello Sarah and thank you for bringing such an important question to the Disney Parks Moms Panel.
I'm so thrilled to hear you are planning your 1st ever visit to Walt Disney World Resort and have chosen to stay at
The Campsites at Disney's Fort Wilderness Resort, which also happens to be my families most favourite!
In my experience of travelling with my own dear Mum who also used a nebuliser and breathing apparatus; on advice from her Doctor we used boiling water to sterilise her equipment where microwaves were not readily available. Perhaps a similar conversation with your own healthcare provider might be insightful and provide extra guidance, which in turn I'm sure will help make you feel more confident about your trip.
However, I am happy to share that Disney Theme Parks benefit from
Baby Care Centres which all have a kitchen with microwave and sink facilities. Baby Care Centres operate during regular park hours and are complimentary for all Guests.
In addition most Disney Quick-Service dining locations do provide microwave ovens for Guests use;
P&J's Southern Takeout is your Resort's main Quick-Service restaurant, however I can't confirm that it does have a microwave for Guest use.
Have a great Disney day.
~ Jane