Based on your description I would take a look at the Class B motorhomes. They are great for a single person or even most couples. They are easy to drive and you can park them anywhere a van can park.
Some have slide-outs which makes for more living space but these increase weight and may reduce storage space. Layouts vary widely and some manufacturers will allow you to customize some aspects of the interior.
The best of the best in the non-slide out B motorhomes are the Roadtrek with the Sprinter chassis and using the 5 cylinder Mercedes diesel engine. A couple I know that have been traveling around the USA full time for the past several years have averaged over 25 MPG with theirs and they have a special rack in the rear that carries a large motorcycle. These RV's have more than 40 cubic feet of outside storage which is a lot for a single person or a couple.
Itasca Navion is a slide-out class B that uses the Mercedes van chassis and engine and would be worth taking a look at as well. With some RV's there is limited access to the fridge and or the bathroom while the slide is in and I personally would avoid one where this was the case.
These class B motorhomes sell for around $130,000 when new but I have seen used ones for sale that had very low use and miles and were 2-3 years old that were priced at $80,000. I would rather save $50,000 and use it for other purposes. One can save $15,000 buying a new 2014 model instead of the 2015 model with the Itasca's.
Check out rvtrader.com as you can select by RV type and find new ones or used ones that are for sale in your area.
Something often overlooked is that the longer the RV the less likely that you will be able to pull off the highway to check out the view or to get into a parking lot at a grocery store or even at a national park attraction, and there will be far fewer campsites into which you can fit at state and national campgrounds.