Splitting into a second comment!
You mention how hard it is in the hospital and I feel for you there, plus there is a huge difference between hospitals.
On this subject I do have concrete advice!
Most specialist hospitals have a play team - they are an invaluable resource so touch base with them ASAP if you haven't already. Most are happy to sit with a child while you go to the loo/shower, sometimes this can be adhoc and sometimes it's something you need to book for a longer session (like to run to the shop).
Can little one be put into a buggy (eg can they be moved from the bed?) As this way you can go down to the canteen or hospital shop (sometimes canteen food is subsidised for parents!) Also if LO is breastfeeding you may get hospital meals.
If LO cannot be moved and there is little option for someone to sit in with them then you need a food stash. Porridge pots, cup noodles/pastas are all cupboard staples - you only need to run to the parents room to put hot water on. Prioritise nutrition as much as possible. Be assertive OP (but still nice!) "I'm here on my own and can't leave LO alone, I'm starving and have to go to the parents room to make some food. Can someone be available to sit in with x within the next hour". If you're seeing different nurses all day they are unlikely to have realised you're there alone. You are a human being, you need to eat, drink and use the loo - that's not an inconvenience or a weakness, it's an irrefutable fact. Our specialist hospital will let you bring in ready meals and the staff will heat them up and bring them to you (parents aren't allowed in the kitchen). But you only know this by asking.
For showers there is often a children's bathroom on the ward with a bathtub in it - these are generally much bigger and can accommodate a highchair/buggy. These are often empty in the early mornings/evenings. The ward should also have towels you can use.
If this isn't feasible (and sometimes it's not) then I take multiple sandwich bags, flannels and bar soap. Pull the curtains round, wet the flannel and have a refreshing flannel wash, put wet flannels in sandwich bags. And use good, cream stick deodorant and dry shampoo - aveda do a powder that you just shake on so you're not spraying it into the air around the children. But my daughter is respiratory so ymmv!
Finally many of the specialist hospitals have a hospital charity, either for the whole hospital or just the children's ward. Contact them and they're very very likely to be able to offer some support.
My last bit of advice is to try and realise that this is now part of your life, but I promise it isn't all of your life. We have found joy, fun and love over the last 2 years that we would never have experienced otherwise. It probably seems impossible today but good things will come too xx