Your son is in that halfway state - he's an adult and in college, but he's living at home like a high school student. You all have to readjust the ground rules and make some accommodations.
If he were living on a campus, you would have no idea where he was, what he was doing, who he was with, how much he was sleeping, or whether he was doing his class work. You'd be going through withdrawal in some ways, but he wouldn't be in and out of your life every day or every 3 days. You'd miss him, and you'd somehow come to terms with the fact that your job is pretty much done except for paying his college bills and seeing a ton of laundry at Thanksgiving. But that's not what happened here - he is still living there, but trying to live a big of a working college student life.
So, I think you sit down and work out who pays for what, what things you will do and what you won't. If he wants to be on his own for meals, then fine, he's on his own. If he comes home and there is extra, he's welcome to a plate of it. Otherwise he can make himself something (because you are DONE being his chief cook and bottle washer) - if he does not know how to do basic cooking, then he's missing a major life skill that you and your husband should have taught him. Or, he can make a PB&J sandwich or a salad or a bowl of cereal. But if he doesn't let you know he plans to come home, then you aren't making him a meal. That means you stop texting him that there's a plate for him in the fridge, and you stop asking him if he's coming home.
Do invite him to his siblings' games, not just so he'll feel a part of the family, but so that he is connected to them. You can set up a main calendar in the kitchen if you want, or use a calendar app like iCal (we have individual calendars, work calendars, and a family calendar). Enter games or holidays/birthdays on "Family" and your oldest will get alerts of events. If he comes, great - but if he doesn't, remember he wouldn't be doing that if he were away at college.
I don't understand why it took days to tell him that your aunt had passed away. If you didn't want to tell him via text, then you could have texted him that you have news that he needs to hear and to please check in.
Don't interfere with brothers jockeying for shower time. Your son is an adult, and your younger kids can learn to share a bathroom without you being the referee. I don't understand from the few details you posted why you were so upset about this. Don't siblings argue now and then about who needs the shower, bathroom, TV, car and so on? They can learn to communicate their schedules so that one knows ahead of time that the other needs to leave at a certain time and needs a shower first. That's called manners. It's what your older son would be doing if he lived in a dorm with roommates.
Re the truck tags - were you reminding him because it is his responsibility to pay/renew them? Or because you were going to pay it and wanted him to know or say thank you? If it's up to him to renew them, don't remind him - put things in his name. If he ignores the reminders and gets a ticket or a surcharge, oh well, that's his problem. He's an adult who doesn't report in about his whereabouts, so he doesn't need his mother monitoring him. He can learn the hard way, or he can take care of it and feel accomplished and adult. His choice.
Your son may be overworked, or he may be frustrated that he's in limbo while so many of his friends are off at college, an experience he didn't choose. He may yearn to be on his own but, as you say, he can't afford it. Now, he could afford it sooner if he stopped buying so many things on line, but it's his money and his problem. What you need to do is have a sort of contract about what's "included" in his rent-free living, and what's not. When my son was here after college and before he found an apartment, he was in charge of all his own stuff like laundry and food, but we didn't ask him where he was or if he was coming home. We learned not to worry and not to meddle. If we had a special occasion, we invited him and expected an RSVP, just as we would have if he lived elsewhere. He was smart enough to be around for birthdays, Mother's/Father's Day and so on. If he was planning to be home for dinner, he let us know, and we either made enough for him or he fixed his own stuff when he got home if I hadn't shopped.
I think you also need to define what you mean by "respect us" - answer it for yourself, and make sure he understands. Get him to agree or at least say where he disagrees. What you feel is disrespectful, for example, may not be - skipping a child's game or telling little brother to get out of the shower is not disrespecting YOU. But texting you that he wants dinner IS - he's either an adult or he isn't. Sleeping late isn't disrespecting you - it's a natural outcrop of a kid with a job, a college workload and a small social life. You have to let go more, and he has to be more independent with reduced expectations of the same mom his little siblings have.