How do you set fused map cache to true at ArcGIS Server 10.2.x? It used to be on the advanced button on the cache tab but I no longer see it at 10.2.
Fused is the default when you building a cache at 10.2. Multi-layer caches are no longer supported. See the following link under 'Migrating map and globe caches':
Map and globe caches that you created in ArcGIS Server 10.0 and previous versions can be used in later versions. The only exception is multilayer caches, which are not supported after version 10.0. If you have a multilayer cache, you should rebuild it as a series of separate fused caches
Actually, all caches are fused at this release. However, we found a wierd bug. If we changed the default name of Layers in the TOC for that dataframe to something else other than "Layers" and then published the map service, it would change the variable "Single Fused Map Cache" in the xml for the rest service to false.
At 10.1, MXD based services were removed from arcgis server. Since Multilayer caches are not available with MSD-based services the option was removed.. Single fused cache is the only option for 10.1 and later.
I've moved your post into the Arcgis for Server space. You will get a much better answer here as the GeoNet Help is intended for community help and feedback. I have also added in a few additional tags. You can see moreon the community structure, and what topics are under each space from the following documents:
GeoNet Community Structure
ArcGIS Discussion Forums Migration Strategy
Retrieving data ...