We live an an area that has amazingly hard water with lots of deposits. Like you it doesn't come off easy and we have all been fighting this for years on our boats here in Havasu!
There is a product available called "Just Ducky" that absolutely takes off the deposits without any issues on either fiberglass or plastic BUT it costs like $12 a 32 oz bottle.....whew!! A friend here told me to add some Lysol Basin tub and tile cleaner to my water when washing and amazingly it works as well as Just Ducky.......interestingly enough it smells EXACTLY like Ducky.
We've been using this for years on our boat (which by the way costs like 4X the cost of a new Jeep) and it works phenomenally well....we actually make our own "Ducky" with it by diluting it in a spray bottle and keeping it on the boat. I've used it on cars and now the jeep as well and it works as advertised.....give it a try and you won't be disappointed.
PS.....we buy it by the case at Costco and we by 4 large bottles for around $8.....much less than anything else out there.