fresh water tank.. needed?

If you're plugged in all the time, and don't want to consider the what ifs, then yes, do away with all the holding tanks ( water, black, gray), and become a mobile home.

I would want a water tank. We often stop for lunch, bathroom break, change a child's diaper, etc. It's nice to be able to wash your hands, flush the toilet, get a drink, get the dog a drink, etc. I always travel with a few gallons of water on board just in case I need it.
We also do a lot of camping at out of the way places that don't have hook-ups. I'd hate to not have that option.

Good point. But if/when we decide to sell, we should be able to add it then. I would hope :-) no plans to sell at least until our kids are older and no longer want to go with. The oldest is 5, so we have a few years.

As long as you want to camp in private CGs, such as KOAs, etc., you probably won't need the tank - unless you want to camp well off-season when temperatures are likely to go below freezing at night. In that case, you would need to fill the water tank and disconnect the hose, in order to keep the hose (and CG faucet) from freezing up. CG owners frown on customers who let their faucets freeze up. :) If you want to stay in state parks (and Wisconsin is one of the best states for them), then you will need a water tank, unless you want to tote & haul by 3-5 gallons what you will use for cooking & washing. Good point already made about selling it.

It's not clear what kind of trailer you have. We had a small popup for many years and had so many problems with the check valve in the water system (which was a really simple system, with no water heater and only one sink, no toilet), that we eventually just disabled the water system. We used thermoses instead, but that was only for washing a few cooking pots & utensils, and brushing teeth. We did leave the tank in, however, so when we sold it, it could be reconnected and used once again.

Bottom line: I wouldn't buy a trailer without a water tank.

Sooner or later you are going to want to sell it. Who wants a TT that does not have a water tank? Severely limits where it can be taken.

Just thinking ahead a little.

Not going to need for potty. Husband has decided to go with a dry flush toilet so he doesn't have to deal with black tank and draining it.

you can probably get by without a water tank, but it comes in handy for those potty breaks en route. you need water to flush. Just install a smaller tank if you are trying to save weight.