We've been RVing for about 45 years and never used a water softener, but have one at home on the hot water said. On our current coach, I've been finding that I'm getting enough calcium deposits to actually clog two of my sinks. My shower and main sink sprayer have the silicone tips and I can just rub them to get the calcium to break away.
With all that said, I finally broke down and bought the "On The Go" double standard size. I was amazed at how heavy it was empty. I'm BIG on making connections at a campsite quick and easy. I knew it wouldn't be long before I would quick using the tank if I had to drag it out each time.
I ended up mounting it in a bay where I can easily remove it to service it. I used quick connects on the water connections. This week will be our first trip with it. I know the showers, washing machine and dishwasher will work better with it.
Do you have to have one, no, millions do with out them, but they're nice to have.