Bob, think Kenworth.
Brandy
Bob;
As I recall, when I wrote the off-highway truck manuals for Mack Trucks (mid-1970s), I had a Cummins Engine catalog which showed applications for their engines. Some RDCs used a special horizontal-mount Cummins six-cylinder engine.
Check the toy stores for a toy semi-tractor with sound. You may be able to add a better speaker to the system and tweak it for your RDC.
Yours,
Dave
have you tried soudtraxx sierra gp7/9 sound? check it out @www.soundtraxx.com, go to their sampls and try them.