Sure! Here's a simple RPG game implemented as a single HTML file with JavaScript. Please note that the game mechanics and graphics will be very basic due to the constraints mentioned. The game will have 20 different enemies, 1 boss, and quests. It assumes smartphone operation and uses pictograms for representation.
Please note that this code is just a basic template for an RPG game and does not include the full implementation of quests, combat mechanics, or more advanced features. It is meant to provide a starting point for further development.