I would never pay for a PDI on a used RV from the selling dealer. When I bought our Class C, I took it to a reputable and recommended RV shop and paid $250 for a complete house and chassis inspection. They checked all the systems and gave me a detailed report with their findings.
We also found lots of older rigs with low mileage. The one we finally bought was a 2008 with 40K on it, considerably higher than a lot of others we looked at. We found that the ones that were used more had fewer issues than the lot queens and also the interiors were generally in better shape.
5 months later and no regrets yet!