Any suggestions on how can I get honest feedback from hotel managers regarding an advertising channel that I want to build? Google ads? linkedin?

You can use LinkedIn ads to focus on your specific target audience based on things like: industry, job title and function.

Facebook also has specific categories regarding job types, I'm not sure if "hotel managers" is on the list.

Best of luck! Let me know if you have any follow-up questions.

The best survey results for such a targeted audience are best accomplished with direct questioning. I would recommend the following steps:
1. Create a survey, whether it's online (surveymonkey) or just a list of questions in Google docs.
2. Compile a list of about two dozen hotels you want to target.
3. Call them and ask to speak to their managers. It might require some effort, but even if you don't get to talk to the person you want you can ask questions along the way.
4. Another route would be to look through LinkedIn and find people working in this capacity and email them directly.
5. Be direct about why you are asking and document everything.

Go face to face in high volume markets and solicit direct feedback. If they're willing to give you time and make a booking, there's your honest feedback. For advertising sales, remember hotels get 5 phone calls a day from salespeople, so sometimes you need a unique approach to grab attention.