How do I determine the optimal Wi-Fi Range Extender to pair with my modem (D2200D)?

Agree with FURRYe38 on the minimum 30 ft.  There are a bunch of different extenders that can all have different features. Pick one that matches what area you're trying to cover and the features you want. I'd recommend a tri-band extender so it has the extra backhaul for usage but they're more expensive. Everything has a trade off. 

Once you have one picked, In my experience, it takes a little trial and error to get it working the best. The best it to speed test it in several locations. If you stick it in one location and walk away, you're probably not optimally placed. HVAC, Concrete, and other interference can change greatly even by moving several feet one or a room over. go by the general guides and test it in a couple locations. And even thats a loaded statement as you can play with distances and then play with Netgears fastlane setup to set it up the best for your configuration.  Take a couple hours and play with your extender and you'll have better results and satisfaction then just putting it in one place and forgettting about it. 

Only other thought was sometimes RV's have issues with signal. Reason is the metal cladding/covering does a great job of blocking signal. Something to keep in mind that it doesn't matter how much you boost the signal if you're basically in a faraday cage.