I also think you are being unreasonable.
My father left when I was 5, I saw him every weekend. He collected me & my siblings, took us to his house & made sure we were home at x time. No overnight stays as he lived in a bedsit.
I cried easily for the whole night/next day after I saw him. I hated it & I missed him like crazy. But we weren't allowed to speak about him in 'mum's' house, unless she asked, and she rarely did. I never dealt with missing him, I wasn't allowed to.
My point is that your dd is more likely to feel upset by her dad having to go home again, not because she has to stay in a different hotel. I would think that the hotels seem more like an adventure than anything worrying/upsetting.
By demanding he have her back at 6, you are cutting at least 12 hours off their contact time. You are preventing him cuddling up to her while she sleeps, or watching a DVD together, or a late night snack, or simply, having a late night together. The bath will be rushed, as will the story, in his desire to get her to bed at YOUR appointed time, and not to out stay his welcome/feel uncomfortable in your home.
By asking him to not have her overnight, you are preventing him from being able to comfort her if she wakes, soothe her if she has a nightmare, having those wonderful early morning cuddles when the children are so sleepy & warm & content. Preventing them from having breakfast together, lazing around in PJs for the morning. You are breaking up the continuity of your daughter's time with him.
You are preventing her from knowing that Daddy will be there to hold her if she is upset in the night, preventing her from being able to build up a trust in him.
I guess you enjoy all that & more with your dd, so why should her father be prevented from doing so?
If you can't allow it to stand for him, do it for your daughter...I am sure she will find 2 goodbyes in one weekend harder than one.