E-350 and E-450 use same V10 engine with nearly identical 8-10 MPG fuel economy. E-450 has heavier suspension, brakes, and higher GVWR. Choose based on weight capacity needs, not fuel economy.
Your 2015 Vista's V-10 pulsing issue that started after mountain passes in Montana is likely caused by a failing mass airflow sensor, throttle position sensor, or fuel delivery problem. The "Fault received from External Node" code suggests a communication issue between engine management and instrument cluster that may be related.
Montana 3793RD black tank valve is at tank outlet near dump connection. Access through underbelly panels. On 2023 unit, likely warranty covered - contact dealer before extensive DIY repair.