country coach aqua hot -- boiler coolant?

The coolant level on my aqua hot is low (it is basically empty in the reservoir tank).

Two questions: what coolant do i buy (it says it requires some special boiler coolant). I am hoping there is a place online (amazon?) that can just ship me a bottle of what I need

secondly, is there any routine service that should be done? the unit has never been serviced in 7 years. Do I drain the old coolant or anything or just top this guy off and I am done?

thanks for your help