Two 12v batteries in parallel is perfectly fine. I'm not familiar with your particular rig but this would be the stock configuration for most Winnebagos.
As far as the converter goes, unless it's been upgraded, you should replace it with a modern converter with a multi-stage converter. The old ones aren't as sophisticated in their charging methodology and can "cook" your batteries. You may need to replace your batteries as well.
Here's what I used to replace the converter in my 2002 Itasca:
In terms of size, there's no harm in going with a somewhat higher amp rating than your existing unit as long as the physical dimensions are OK. The amp rating is directly related to how fast your batteries will charge. In addition to PowerMax, Progressive is also a well-regarded brand, among others.
Replacement is straightforward although access may be a challenge. If you can, disconnect one wire at a time and connect it to the corresponding terminal on the new converter. If this isn't possible, label each wire as you remove it (DC +/-, AC hot/neutral/ground, chassis ground, etc.)
When I did mine, I disconnected everything and then discovered my rig's wire color-coding was neither typical nor was it what showed in Winnebago's diagrams. i only discovered that I had reversed the DC polarity when I turned on the AC power, heard a pop and smelled smoke. Fortunately the protective fuse blew and no other damage was done.