This one will do two batteries . I have never used this exact one but its cheap enough and digital readout helps you see in .10ths of a volt which is helpful on telling exactly what you have.
https://www.amazon.com/Marine-Voltag...ustomerReviews
Check on your fishfinder menu....maybe it will display voltage and you just have overlooked the setting. I will say though several of my fishfinders are not entirely accurate on voltage. But I know how much they are off and I can tell by the drop over the day how I am doing on reserve.