I'm not a military mom, yet, but my mom was since I was in the Navy, my MIL too since I met my husband in the Navy. Both of my grandmothers were since my dad was in the Army in Vietnam and my uncles on my mom's side were both in the Navy. My grandfather served in WWII also, so we're just a military family way back.
I know that my mom would call me at least once a week to chat and that she sent care packages all the time. My parents also made a point of visiting me at least twice a year wherever I was stationed (more often than that when I was in Hawaii! lol!). I think that seeing how I was living and witnessing my daily life in person put their minds at ease. Also, the hubs and I aren't in the military any more, but we still live 3000 miles away from my family, so we're setting up webcams for video chat so my folks can see the kids more often. Maybe that would help you.
You should know that your sons are being well taken care of by the military. They are sure of getting enough to eat and always having a place to sleep as well as constructive work to do that can make them proud of themselves. Getting "enough" sleep is another matter and depends on what they're doing, but they are young and strong I am sure, and going a little short on sleep isn't going to do them any lasting harm. Yes there are other risks associated with military service, but your sons are adults and have made those decisions for themselves. They know that you worry about them, but probably hope that your pride in their service will help to make up for some of the worry.
Anyways, your worry and anxiety are understandable and I'm sure very normal, especially this day and age. Just remember not to burden your sons with it. A strong and supportive mother is the best friend a young single guy in the military could have. :)