Ok, so I read the whole thing, twice. I had to look at it from your point of view, and then also put myself in your shoes and act like it was my in-laws.
I've been in your shoes, and you need to get out of your shoes. Throw those shoes away, get some shoes that are made for ALL activities. Am I making any sense? Sometimes you want to wear heels, but you need to have some tennis shoes in your closet, too. Dust those thangs off, and go camping. Personally, my husband and I used to go camping all the time, and it was okay. (Not super awesome spectacular and not awful. Just okay.) We also went just the two of us, before we had a child and without family.
Our daughter will be four in September and we still haven't gone camping since she's been here. It sounds awful, lol. I don't even want to deal with the heat and bugs, much less force my small child to. I don't know where you live, but the middle of summer is the last time ever that I'd choose to go camping. No thanks. Also, I would never go camping with my in-laws. Most likely for different reasons than you have, but still, in-laws and camping, no.
My mother-in-law likes to take my child off to do things where I can't see them, and it bothers me greatly. While she has had a small football team of children, she also let them catch her house on fire, break their faces after falling off the fridge, drive a car into the house at the age of two, and left each of them at the store at least once, sometimes not even noticing until they'd walked all the way home AS SMALL CHILDREN. Do I think she's a bad person? No. Do I want her alone with my child? No.
Not to mention, one of the three children she has living with her has been diagnosed with bipolar disorder and schizophrenia and recently assaulted my husband, kicked in his mom's door, and found out he has a permanent STD, and I just don't want to go camping with him. End of story. With that said, if my husband's family called and wanted to go camping I would put my foot down and say no. It would be easy for me though, because my husband would say no before I even had the chance to.
Now, take the crazy brother-in-law out of the picture, and I would have to be nice and say yes. I would hate the stress of my mother-in-law who let her children jump into the deep end of pools at the age of two taking my child off to the lake, and I would certainly not let her do it, but, if the worst thing happening out there is that I'm standing up to my mother-in-law and telling her I want my daughter in sight at all times, then we're in good shape.
Bugs suck. Take bug spray. And, Google easy camping recipes. There are numerous sites where they give you great instructions on how to prepare meals before you even leave, or organize pre-cut ingredients for easy assembly. Tacos aren't the only easy camping food out there. It sounds a little bit to me (and please don't get offended) that his family and you, are both being stubborn. She doesn't want tacos and you don't want to make anything else. Well, she should suck it up and eat them for one measly meal, and you could just make something else. You know?
Even funnier, if you ended up making something else that she hated even more! But, that wouldn't be the mature adult thing to do. *sigh*
Your husband, on another note, needs to put his own foot down. You and your child come before anything else. Your child's game is more important than a few hours of sitting around getting bug bitten in the sun. By far. If my husband tried to pull that shit, he'd be unpleasantly surprised to find that my daughter and I didn't show up for camping
at all. He also needs to set aside time for his dad's calls. Not during your time. If his phone rings during a movie, he needs to silence it and call back when the movie is over. If he thinks it's important, he should answer and when he finds out it's not important, let his dad know he'll call back when your movie is done.
My husband used to have this same problem with his mom. He could
not tell her no. We've been through many very awful and tough bumps in our marriage thanks to his mom and his gross child-like attachment to her. But, we've also pulled through them, learned a little about ourselves, and changed accordingly. You guys sound like us, three years ago. Keep your chin up, you'll get there, and he'll get there. It's a matter of stepping outside your comfort zones. You, going camping and
trying to enjoy it. And him letting his dad know straight up that you and your son are first and that he can't always be barging into your lives.
That's the first step, and I know because we've been there. It's hard. And it's really easy for people on the outside looking in to give you "advice". It's a lot harder when you've been dealing with it for years and you don't want to hurt anyone's feelings or deal with any confrontation or cause more problems in your marriage. I know. I really do. It's important to find some alone time where you can sit down with your husband and voice your concerns and irritations
and set rules, that you can both agree to and abide by.
It sounds a little bit to me like you have a lot of bitterness towards his family (not wanting to go camping and finding every reason why, and not wanting to go to the reunion) more than likely due to years of biting your tongue and getting walked all over not only by your in-laws but also by your husband
because of your in-laws. I know. Know why? Ding ding ding! Cause I've been there! It's a shitty situation, but it isn't going to change until you and your husband are on the same page and he rips that page out and tapes it on his family's metaphorical forehead.
As for making the week pass, definitely try being nice and civil. If you're already grouchy and bitter before you leave, it's just going to make your time, your son's time and your husbands time awful. And that isn't fair. Sometimes the LAST thing you want to do is grin and bear it, but you're a strong, mature mom. Grin and bear it.
Take some of your child's favorite things to do, a bug holder and a net, a spyglass, set up horseshoes, take some books, a notebook to write or draw, or maybe even branch out and take something entirely new. Maybe try knitting or something?
I hope everything works out and that you can all get along and have fun, but more importantly grow as adults
together in this time!

Good luck!