Wireless sensor networks (WSNs) are considered as a promising solution in intravehicle networking to reduce wiring and production costs. This application requires reliable and real-time data delivery, while the network is very dense. The time-slotted channel hopping (TSCH) mode of the IEEE 802.15.4 standard provides a reliable solution for low-power networks through guaranteed medium access and channel diversity. However, satisfying the stringent requirements of in-vehicle networks is challenging and demands for special consideration in network formation and TSCH scheduling. This paper targets convergecast in dense in-vehicle WSNs, in which all nodes can potentially directly reach the sink node. A cross-layer low-latency topology management and TSCH scheduling (LLTT) technique is proposed that provides a very high timeslot utilization for the TSCH schedule and minimizes communication latency. It first picks a topology for the network that increases the potential of parallel TSCH communications. Then, by using an optimized graph isomorphism algorithm, it extracts a proper match in the physical connectivity graph of the network for the selected topology. This network topology is used by a lightweight TSCH schedule generator to provide low data delivery latency. Two techniques, namely grouped retransmission and periodic aggregation, are exploited to increase the performance of the TSCH communications. The experimental results show that LLTT reduces the end-to-end communication latency compared to other approaches, while keeping the communications reliable by using dedicated links and grouped retransmissions.