Why do you create a different / RWD design for a tablet?
Not to mention the UI people are familiar with on the phone is slightly different than the tablet and it's nice to present something more familiar for each format to reduce the learning curve of the user and make sure they have a higher success rate using your site on any platform. I've tried a couple of methods and the jury is still out on which I prefer but they both seem to work well in all formats which is all that can be expected.
Designing a site for style over function or cutting corners just because you don't want to test a site in 3-4 layout modes might make your development time lower but it certainly doesn't do your visitors any favors. That's kind of why I'm working with templates where someone else has already went to the trouble of making a very complete and robust RWD template so I can focus on the content and SEO aspects of the site and not worry about such issues of scalability and usability on all the platforms because it's already been done for me by the template right out of the box.
Obviously what I do as a one man show is different than what a team would do but in my situation there's working smart and working hard, I prefer the former as it tends to yield maximum results :)
FWIW, I've got one little site I did for some remote admin code that presents itself as a normal webpage for all platforms but when on a cell phone looks just like an app with an icon menu etc. and it really functionally rocks as a cell phone looking app but that approach which seems intuitive for the little device seemed cumbersome for bigger screens.