I would also suggest calling the factory.
Ask them *why* an extension cord voids the warranty. They will likely tell you its due to people using low-grade cords that have too much voltage drop. Then ask them what wiring they'd recommend for your setup (gauge, etc.). If you extension cord meets or exceeds their specs, ask them how/why that voids your warranty when it is functionally equivalent to a "properly" wired setup.
However, if you're "in the business", you should know that extension cords are prohibited from acting as "permanent" wiring, which is effectively what you're doing. Regardless of what the dealer said/did, I think YOU know this is an improper setup on a technicality.
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