There is a distinction that you (and your professors) need to make: A gap year taken to better yourself or others is not going to look poorly in admissions. It will show personal growth and/or a desire to translate your abilities to your community. A gap year taken, during which you don't do anything productive toward your field (other than for financial issues and the like), may come off as more lazy, but can usually be explained.
Many professors, graduate students, and post docs talk about how candidates who have taken a year (or more) off, are generally more mature and able to handle stresses of graduate school. Of course, you can still be mature without the gap year, but the gap year can give you more experience in ways that academics can't.
Taking a year to volunteer with AmeriCorps is not laziness or being sick of academia. It's a desire to help your community and gain experience translating knowledge to others' needs. The benefit of having more time for your application is great, and is not lazy, either. See if you can get something as close as possible to your desired study field while working with AmeriCorps. If that's not possible, that's still fine.
If you have prior research experience, then you should be fine for your applications. Think of it this way: If you apply next year, after doing service, you'll still have the same amount of research that you would have been applying with this year. And you may already have a significant amount now. Service will only strengthen your application.
I am not 100% sure I am taking a gap year. I, like you, am interested in service, specifically Peace Corps. My plan for the year is to apply to my top 3 PhD programs with all I have. But, I am also applying to Peace Corps. If I am not accepted to any PhD programs, I will begin my service in Peace Corps around the same time that I would be starting graduate school. Peace Corps is 27 months of service, during which I hope to be placed in environmental education. I feel like this could only strengthen my application, not weaken it, and I would gain experience that most people never get.
You are only a senior undergraduate (like me), so you have time. Many people take time off between undergraduate and graduate education for various reasons, and that is fine. Your professor seems to have tunnel vision for academia, and he may not realize that not everyone is the same. Some people want to take time to explore service options, as well.
Good luck in your service, and I hope you get accepted to a great program once you apply.