If there is a water filter in ours I am unaware of it. I have had the basement panels down fixing the hot water tank leak and didn't see one in that area. Pretty sure there's not one under either of the sinks.
Yes, the odd storage bay with the electric outlets. I don't know what the purpose of that was intended for. I use that area to carry my cast iron cookware and burners. Also carry a couple of extra bag chairs in there.
The fireplace feature is nice on those cool evenings when you don't really need the heat on or if you want some additional heat. Works much better than I thought it would.
We put a small fridge and a wall mounted TV on a swingarm mount in the basement storage area since it was prewired for TV and had the electric outlet as well. Also put LED strip lights all the way around underneath for accent lighting and for better visibility at night when walking around outside. That's probably the best $80 I ever spent.
I have had the belly panel part way down as well and can tell you there is no insulation in there. I am planning on correcting that this spring. Will likely add a ceiling fan over the island at some point too. The bedroom air is loud but it blocks all outside noise so we actually decided we like it that way. Others may have a differing opinion on that.
Let me know if you find a water filter and where it is located.
Good luck. Not sure where you are located but we found the best deal near us at Colaw RV in Carthage, MO. I found a slightly better deal in south Texas but it wasn't worth the drive to go get it.
The journey is as important as the destination.