I say absolutely NO to the sleeping together idea.
I went through this with my stepson.
He wanted to bring his girlfriend to stay at our house and she was a great girl and I liked her, but I put my foot down about them sleeping together at our house.
My husband got mad and said he'd get them a hotel room then and I put my foot down about that too. Was he bringing her to visit or just to screw her? I know that sounds vulgar, but I didn't want any part of that. She was only 16 years old.
So....they came and stayed. She slept in my daughter's room, stepson slept in our office on the daybed and we had the younger ones share a room.
Keep in mind I had a daughter who was 11 and a son who was 1 and there was no way that precedent of sleeping together in my house was going to be set.
My husband and I had a huge fight over it, but you know what? The kids happily complied with the sleeping arrangements and we all had a nice visit after all.
Just because you have a small camper, it doesn't mean the kids have to share the same bed. Not at all. Your husband might not like it, but there is an alternative and I would certainly insist on it.
That's just my opinion.
I'd like to point out that what turned my husband around was the fact that he was thinking about his son sleeping with his girlfriend. I asked him if he would allow our daughter, at 16, to bring home a boy and sleep in the same bed and he said HELL NO! So...again, we needed to head that argument off at the pass well before the fact by not allowing any of that in our home. He agreed.
I'm a bit surprised your husband would want his daughter sleeping with her boyfriend in front of you on a family camping trip.
From my experience, if you say no, the kids will be fine with it.
Best wishes.