Replace failing N3150 with NovaKool R4500 compressor for better heat performance or Norcold N3141 for absorption drop-in. NovaKool better for boondocking with solar, absorption for propane versatility.
For your 2005 Southwind with 400W solar and 4 6V batteries, upgrade to a 2000W pure sine wave inverter like the Aims Power or Renogy models. A 2500W unit may strain your battery bank unless you upgrade to lithium or add more lead-acid capacity.
Two batteries using more water indicates failing cells or connection issues. Check terminal tightness, measure voltage during charging on each battery, and test specific gravity. Higher voltage on some batteries indicates internal resistance. May be warranty issue at 6 months old.
LiFePO4 offers longer life and lighter weight but requires checking converter and alternator compatibility on 2003 Terra. For light use replacing lead acid every 2 years, break-even takes 6-8 years. Consider if usage pattern justifies higher upfront cost.