There is one situation in which your Parallax 73XX converter can be an all-right charger/maintainer for a certain kind of RV battery:
Keep the Parallax .... and instead use the money you would have spent to replace your battery with one that is specified to require the same voltage for storing (floating) it as the single fixed voltage Parallax puts out - and will also charge good enough for many camping styles(ies) using only the single fixed voltage as the Parallax puts out.
i.e. We have had only a Parallax 7345 converter in our RV for over 10 1/2 years. But we changed to AGM batteries that are specified to be stored (floated) on 13.5 to 13.8 volts. In addition, since AGM batteries have much lower internal resistance than liquid acid batteries - they will also charge fast enough (for some camping styles) using only the Parallax's output voltage.
It was a no-brainer in our case because we wanted to change to AGM batteries anyway, for other reasons. We just had to look for AGM battery models that could be stored months at a time on around 13.6 volts without being damaged. We move a lot on our RV trips, so the engine alternator brings them up to full charge in just a few hours between camping spots if we have not been on hookups or generator power long enough at the last camping spot for the Parallax to fully charge them.