The only one I've ever had a hard time finding was on Marc's (Twogone's) van, and they were on both rear sides deep down under stuff. His is an older 7.3 diesel though, not sure what the original floor plan was. Like Rallypanam said, if they aren't easily visible on the frame rail between front and rear wheels then they are probably behind the rear wheels on one or both sides, in boxes and accessible only from inside the van.
We have two 4D house batteries on our 03 EB diesel ... One behind the 2nd starting battery along the rider's side rail and one under the rig behind the rear axle but in front of the spare. Joe
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Joe
2003 EB50 7.3L PSD Q4X4
2000 Chevy Express 3500 High Top EB37 - Sold
2003 EB30 - Sold