1. I'd get both the engine and the generator's oil changed--or have the dealer get it done as a condition of the sale.
2. Its the same as a Class-C as its the same E-Series chassis underneath
3. Only if you want a brand spanking new unit with all its problems; presumably the previous owner fixed everything (or got so disgusted with it they dumped it). Note that the Axis/Vegas lines have seen a bit better quality than your average Thor unit..not sure why but that is what appears by all the posts on this forum.
4. I don't know.
Want to see what a 25.1 can do, check out
48 in 366 they are taking a 25.1 to every National Park in the lower 48 all through this year (and visiting many many State Parks in between). Lots and lots of miles going on that unit...