Hi Team185,
We have a 2016 NT 24BHS and did a couple items to customize it for our needs. We're a young family from Wisconsin, with 2 little kids so storage is always needed. We use it pretty often, at least 50 nights per season and includes usually two 7+ night trips.
You're right, there's a good deal of wasted storage under the sink and bunks.
Under the sink we removed a back panel of flimsy paneling. This panel protected(?) some basic heating & plumbing pipework, so we built a simple platform to cover these items which give us a bunch more storage here.
Under the bunks is a big open internal space that'll give you the storage space of about (3) 27 gallon storage bins. Taking a peek was simple, just pull aside the bottom mattress then unscrew that plywood. You'll find some additional heating/plumbing pipes along the floor. We built a simple platform to cover these then boxed in a storage area and finished it with T&G cedar & 12v under cabinet lighting. As an afterthought, it was a big gain for storage, but its a pain to access so we use it for overstock & emergency items we don't need often.
We also replaced the POS short queen mattress with a much better one, from Amazon. This was a nice upgrade but due to the height of the new mattress and the curvature of the front of the camper this resulted in the loss of those silly sliding doors. Not a big loss and worth the investment. BTW, that stock mattress is about $20 worth of foam from Walmart - that's it.
As for the bathroom, we replaced the shower-head with a much nicer unit, and it's so worth it. Same for the stock shower curtain.
The TV was also replaced with a larger smart tv. With this, we add a hard drive packed with movies for the kiddo's.
We've also found that rarely use the dinette so long ago we removed that table and the floor mounts since they just got in the way. Our Spring 2019 project is to remove the dinette altogether and put in a reclining couch.
We're also planning on mounting a light-weight truck box on the pull-out bike rack. I wouldn't put much weight here so the plan is for storing kid toys like swimming noodles, soccer balls, etc.
Happy Camping!
Rob