If it is a chevy/gmc chassis it is mostly the 3500 version (definitely if it has dual wheels). Doesn't really make any difference for a battery. Get the biggest battery your budget can afford that will fit in the space available.
In 2004 the 23A was available on the Chevy 3500HD chassis. The 23W model, with one slide, was available only on the Ford chassis. I don't think there were major changes between 2003 and 2004.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
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