It may also be that she's playing down any fun she has so that her mom won't feel displaced. I would not bend over backwards. My SD would tell her mom that we weren't fun (and face it, not every day will be fun in a regular family) and we just had to filter it through 11 yr old eyes. It's very common for kids to need transitional time from one house to the other, especially if there is any conflict between parents. Give her the time. You may only get a few good days but frankly, even when SD was living with us, not all days were sunshine and roses. We took the kids to EUROPE and they were grumpy some of the time. I would just ignore it.
Your DH should just calmly state that everyone is going camping and she'll probably get over it once she's there. When my SD was 11, she would come home sobbing and the long and short of it was that she missed each parent when she was with the other one. We assured her that she could see/talk to her mom and we did normal things. I would not fawn over her. I would give her a little space, be normal and consistent.
I would also let her dad take the lead. Something many SMs get into (myself included) is this desire to make it all one big happy when many days it is 1. not and 2. not our job.
I would not try to cajole her mom. I would just have DH state to your SD/his ex that it's his visitation weekend and he'll see her on x day. SD doesn't get to dictate visitation with her dad over something like this.
This is a transitional year. It was the year both my sks started to see things from their own perspective, not just what other people told them, and for SD it was a very difficult thing to navigate.
IMO, I have no issue with bonus son/daughter/mom per se, but I also stick to stepchild/stepmother because I want to take the negative out of it. It is who we are and it is an accurate reflection of the relationship. I think the more "good" stepfamilies that people see, the less stigma there will be about it.