We own the next size up Phoenix Cruiser, a 2350 with a slide. Given your desire to tour in the RV and visit National Parks and perhaps state parks as well, I certainly think a PC 2100 with a slide would work well for you.
Our chassis is the same as a 2100 and we have the V10. It is a great, trouble free motor and you should count on 10 mpg with the rig and smile because you will probably get 11. You will, of course, not need to tow anything.
If you need more storage space, slap a hitch carrier on the back of the 2100 and you will have it made.
As to the slide, slides vary in quality from brand to brand. I can tell you with certainty that our slide in our Phoenix Cruiser does not leak, operates perfectly and is very well built and engineered. Many slides (when inspected closely) should and will cause problems because they are poorly designed and crafted.
Another poster on this forum (and the PC forum as well) is GARMP and he has a thread here down the page a bit. He recently purchased a new 2100 and might be a resource for you for information. PC designs, like some others, tend to stay pretty constant through the years so a ten year old unit and a new one will have a lot in common.
We researched small B+ motorhomes for two years and decided on the Phoenix Cruiser because of its reputation for quality. Read the forum on their site and you will see a happy bunch of owners.
A slide on a 2100 will result in you having a small, easily driven and parked coach with reasonable economy, a couch that makes a pretty nice bed, a dinette, a nice dry bath and a nice kitchen. You will also, most importantly, be owning a product produced by a company that the folks that designed your 2100 own and work there every day. These things are not mass produced and the company only makes a little over a hundred coaches a year of ALL sizes combined.
You will have to search a bit on the various seller sites including rvtrader.com, rvt.com, the Phoenix site, etc. to find a previously owned 2100 and they are out there. You will also need to be willing to travel a bit in all probability to buy one.