Disaster area map generation and sharing are critical to disaster response operations. In post-disaster contexts however, cloud-based mapping services and data may be unavailable because of network challenges. Disruption Tolerant Network (DTN) architectures have been proposed for data sharing in challenged networks. However, map generation may be too complex for individual DTN nodes given their limited computing resources. To generate and share maps of disaster areas, we present DTN MapEx, a distributed computing system for mapping that operates over a DTN. DTN MapEx distributes disaster map data and map generation tasks to multiple nodes to minimize individual computational loads. In the system, responders and volunteers act as mobile sensing nodes. They log the GPS traces of their traversed paths and collect disaster area map data such as the coordinates, images, and assessments of points-of-interest. The mobile nodes then route their collected data and a task request through the DTN to pre-deployed, fixed Computing Nodes. The Computing Nodes aggregate the data to generate a map and opportunistically route it back to the network. To reduce complexity, mapping tasks and data are divided amongst Computing Nodes based on their current computational load. Computing Nodes periodically update the DTN about their current loads. Mobile nodes use these updates in deciding where to allocate their task requests and data. In this paper, we present the design of DTN MapEx and perform initial evaluations on its feasibility in disaster scenarios.