Multiple AD domains for different hotels of the same chain?

This will not be an easy answer as there are a lot of things to consider. But there are few key arguments within your post.

.. as they want each hotel to be standalone so that when it gets sold to a new owner there is no additional work needed to be done.

This is a valid point and there will be no easy way to do this in one domain. Even in one forest this can be a bit tricky sometimes.

and allows us to share MDT and WSUS servers, something we could not do before.

MDT and WSUS are not domain-bound or authenticated and can be run through multiple domains. You don't need one per Domain.

ad.hotelgroup.com. Is this a good idea or not

Usually it is, yes. If the key feature is "easy separation", it's most probably not.

each hotel has its own domain ... There are no trusts between domains..

This is not a scalable AD architecture.

Even if they did approve and fund it, which seems unlikely, I'm inclined to think that converting this would be beyond the organization's capability and appetite for change. When the decision was made for the current architecture, they essentially pressed the ****-it button and conceded that reasonable technology management and architecture is not a priority.

What you may want to consider is creating a management forest, and configure each of the domains/forest(s) to trust that forest. That would help with some, but not all problems, create minimal friction, and preserve the existing architecture.