Hard water is an issue throuought most of the country. Adding baking soda will increase the alkalinity and , depending on your water chemistry, it might make the problem worse; increased alkalinity typically increases the Langlier index (a measure of the scaleability of water).
In order to stop the spots, you must remove the minerals that cause them. Iron, manganese, magnesium, and CALCIUM ARE THE PRINCIPLE HARD WATER MINERALS. They may be removed with a water softener; however, even the compact one offered by Camping World, is a nuisance to keep regenerated with salt. Further, many people are concerned about sodium intake; sodium is added by a softener in direct proportion to the hardness of the water being softened.
Reverse osmosis is by far the easiest way to treat the water in an RV. When I retired from Culligan and bought a rig, I was disgusted with the equipment commercially available, so I built my own. You can have a 300 gallon per day system for about $600 and make it fully automatic for about $800. Check out www.vagabondwater,com
for more details. I installed an external hose bib on my rig so I could have RO water (almost minneral free)to wash the rig and my car. My shower glass is still like new and I don't itch after a shower.
Another adantage of RO water is that it purifies the water as it removes minerals (it removes parasites such as cryptosporidium and 99.9% of bacteria). RO water is used for industrial and medical applications such as water pre treatment for kidney dialisys.
The Vagabond unit runs on line pressure and has no moving parts to complicate maintenance. Check it out.