Our coach is a 2004 Marquis and its Aquahot is the AHE-100-02S model. The manual for this model does not specify boiler fluid to be used, although later years' Aquahot models do specify the non-poisonous boiler fluid MUST be used. Throughout the 02S manual, all it says is to use the poisonous "antifreeze". The antifreeze can be purchased full-strength and diluted to 50/50 with distilled water or simply buy the premixed 50/50 blend.
Bill Sprague pointed out in a thread several years ago that the antifreeze does not evaporate much but the distilled water does evaporate. So, when the overflow tank needs a fill, then consider simply adding 100% distilled water. To be safe, a specific gravity device would tell you if the antifreeze/distilled water solution existing in the tank is "rich" enough with antifreeze to add 100% distilled water.