using di only is a ver costly way of doing wfp@ approx 80 quid a bag iv certainly
noticed the differance since iv ad the merlin
but iv ad gud teachers along the way jsm (john )
being my major one but as i love trad im easy
aving a minium price per job if round is spread out
also helps , but as mine getting more n more compact iv
noticed the differance using tulsion after RO
lasting ages
Think about this Gav-
I operate a 400ltr van mount, my tap tds is 90ish ppm. I only use 2 x bags of Tulsion per YEAR! total cost for my yearly pure water is £145.
I can fill my tank in 10 mins from any tap, anywhere, never run out of water. No membranes etc to change, no RO to mess with or defrost, no waste water either.
I come home after work, plug hosepipe into tank for 10 mins to fill, then that's it, done! As simple, easy & cheap as that!

Don't be fooled by poorly informed nonsense you can read on here at times, like P@F said, the DI only guys, with plenty of experience will know just how much better this is.