The Parallax 7345 is a single stage converter. It supplies a constant 13.8 volts. At that voltage it charges batteries very slowly and once charged it boils the water in the battery.
It is much better to replace your old converter with a PD 4600 converter. It will charge your battery much faster and then when charged it provides a voltage of 13.2 volts that keeps the battery charged without boiling it. It also provides desulfication of the battery.
As far as the 45 vs 55 A it depends on how many batteries you have. If you have 2 or more the 55A will charge the batteries a little faster.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
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