I’m going to leave this up since it could spark some good discussion but remember that nobody here is a doctor and they’re definitely not your doctor. Before trying any sort of fasting, you need to discuss it with your doctor first.
I’m so sorry to hear about your diagnosis.
Please, take this random thought for what it’s worth: you might try interviewing the 10% of ALS patients, who’ve made it past 10 years and see if there’s a pattern in their lifestyle (diet, sleep, etc.)
I would be very wary trying to manage this with fasting. People may have managed other diseases with fasting however those diseases are not ALS.
ALS specifically is going to be characterised by increasing lack of energy which will be exacerbated by fasting. Without putting too fine a point on it, the reason your nurse told you that eating fast food would not be an issue is because the ALS will kill you before changes from fast food will.
As the disease progresses getting energy in will be incredibly hard for you so while you are able to it would be very beneficial to focus on building strength. Honestly - as beneficial as fasting may be, yours is a very specific situation. Fasting will not cure ALS and to be honest eating very nutritional foods and focusing on building muscle will probably be much more useful to you in the long term.
Dr. Terry Wahls recovered from MS using ketosis and fasting. She continues to research using real patient data and alludes to ALS being a disease that you can manage with dietary changes. The book was very powerful for me and even cried at the end as she went from a tilt/recline chair and suicidal thoughts to riding a bike. Good luck!