If you usually fill soft water from home and top up whilst away you might not cause much damage to boilers etc as next time soft water comes along it will break down the build up of hard water deposits,
I live in hard water area and fill from the softener before each trip,
If i know im going to need to top up before getting home id usually top up with at least half tank of soft water remaining and that will half the hardness of the incoming water,
There is a lad in Galway that works with Softeners that has all the info if you need his details i can post,
My boiler is now 18 years old and never had a problem with scaling up.
AFAIK limescale (calcium) deposits only occur when water is heated above 61°C. The temperature options in a Truma (for example) are 40°C and 60°C so scaling shouldn't occur in the hot water system.
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