Car batteries are not designed to be used as constant drain.
You really do need a proper leisure battery.
They do still wear out though

I'm on my second and I'm having to get a new one, so that'll be my third

But I do have it blasting flat out all the time (the pump that is, I like a high flowrate) and if I have a really busy day then it now begins to fade near the the end of 650 litres.
If it is fading then it means I have ran it down too far, the one thing you should not do with a leisure battery is run it down, and NEVER run it flat.
They are designed to give a constant voltage for long periods of time, if the power is fading in them, then you have taken it too far as it is no longer able to maintain that constant voltage.
If you keep doing it then you will rapidly shorten your battery life.
I've made the mistake of not having a split charge relay fitted, apparently you should still charge up the battery regularly unless you are doing substantial milage.
But I'll bet it is the difference between a battery lasting you 15 months and one lasting you 2 or 3 years
If you want one fitted, check out any local tow bar fitter, they are doing it all the time for caravans and so on and it isn't that expensive to get it done.
Unless you are a dab hand at messing with your van electrics of course

Ian