I then decide to add a reviews page that has a bunch of nice reviews. I set the focus keyword to 'Reviews' (using Yoast-SEO plugin for Wordpress)
What you want to do is make a focus phrase " campsite reviews" instead of just "reviews". You may need a different plugin if Yoast-SEO won't allow phrases as the focus instead of individual keywords.
What happens when somebody searches on Google my campgrounds name followed by reviews?
If the TTFB (time to first byte) of your page is under 200 milliseconds and the content is excellent and the top keywords are
<insert business name here> reviews together in that order spread nicely throughout the page, then that someone could see your page as showing up first, unless someone bought ad space from google about your topic, in which case, they will appear first in the search results.
Google Webmaster tools is your best friend. Visit it and add your website and four days later, visit their search analytics section to see what people search for in order to find your site in the results. That will give you a better idea of what keywords you should use and what order to use them if you want your site to appear near the top of the search results. A higher impression count for a phrase in search analytics means more people are interested in that phrase and that phrase is what you should try to go for then on your site, you can have a common section with a link to reviews so the searcher can still access them.
-Does Google know that this reviews page is from this campgrounds website and therefore shows up high on results?
If the exact phrase
<insert business name here> reviews appears 2% to 5% of the time in the page in the important places such as header tags, then it might rank well, however, google evaluates other websites to see how they fit compared to yours to determine which appears first.
-Or will the focus keyword 'Reviews' only be optimize for that word and be lost a million pages deep for the keyword 'Reviews'?
Trying to optimize for the word reviews by itself is like trying to be better than millions of pages because that word is so common where as
<insert business name here> reviews is less common.
Nevertheless, think about what people want from your site, not what you want to boast about (such as reviews). Try going for phrases like
lower priced campsites at <insert business name here> or maybe
unique adventures at <insert business name here>
And in my answer, replace text in
< > with the name of your business, and try to keep the phrases short as using longer phrases will require a higher word count on your page.