The amount of storage capacity or tankage required is certainly related to your camping style. We rarely spend more than a week in one location. In general a tank of water can be enough. Ginny goes into the water saver mode... paper plates... and so on.
There is a difference between dry camping
and true boondocking
. In a true boondocking
situation without available bathrooms the black tank can become the limit and not your water supply. The two of us can stretch the black tank to 3-4 days. Certainly a single person could double that.
Very often when dry camping
, no hook ups, there are relatively nearby bathroom facilities like when we camped at Craters of the Moon and than the limit is not the black tank but the gray tank.
The water tank is not the limit because you can always add water to the tank it's just a question of the amount you add at one time.
If you are dry camping
at a place like Craters and have to move your rig to dump or fill your water tank, it's not too big a deal once a week. We try to keep our trailer in moving trim, having a place for everything and not leaving a lot out. In general we carry very little loose in our trailer.
As to sleeping on the front couch, it's wider than half the bed I share with Ginny and we have no problem with it. As well I've fallen asleep a number of times on the couch and find it comfortable. We have stored our sleeping pads under the bottom and/or the back of the couch. Presently we use Alps self inflatable matresses learned from Dave and Paula B. We keep ours behind the back of the couch (our couch no longer folds up.
For additional storage we added a closet over the bathroom end of the couch and a drawer under the right side of the couch. Ginny uses two plastic bins under the center of the couch.
If you're traveling alone, the best mod we and others have made is to make our table into a folding table. It really opens up the trailer.
Wishing you well.