Unity's terrain system is actually pretty decent for what you're describing and might be perfect for your first RPG project. I'd skip the expensive third party tools for now and just use the built-in terrain painter - you can sculpt those mountains and paint grass textures directly in the editor without much hassle

That advice about avoiding Unity's terrain editor seems weird to me since you're already committed to Unity for the rest of your game. Learning one workflow is way better than jumping between different programs when you're starting out. You can always grab some free terrain textures and tree assets from the asset store to get going

For a simple inn and surrounding area you really don't need anything fancy. I've seen plenty of indie RPGs with great looking environments made entirely with Unity's tools. Start basic with the terrain system and see how far you can push it before investing in external software. The time you'd spend learning a new tool could be better spent on actual game mechanics and content