Jerry,
Your chassis was definitely built after Monaco purchased SMC. Therefore, since Monaco used the Roadmaster designation for all of the chassis that they built, your chassis is a Roadmaster. However, it was probably built at the Magnum factory in Harrisburg Oregon before Monaco closed it and moved all chassis production to the facility at Coburg Oregon.
There are several features that set your chassis apart from the SMC designed Magnum chassis that was used on earlier model Thunders. The most obvious is the addition of the tag axle for a much higher weight capacity. The only tag axle chassis that SMC used was for the 42 foot Marquis. The other significant change was the installation of Bendix Spicer ADB22X brakes to replace the problematic Meritor ADB brakes used on the 2002 Thunder or the air over hydraulic brakes that were used on the earlier Thunder coaches.
Don,
If you have a 2003 model coach, it has a Roadmaster chassis that was probably built at the Magnum facility in Harrisburg.
Gerald