I would definitely get the new separator. It 'protects' both batteries and works without user effort. Also good if you're parked for a long time with solar.
You could try to resurrect your old house battery if you think it's dead due to sulfate buildup on the electrodes. A pulse charger can actually "fix" a bad battery that has this issue (but not fix it due to other issues).
My favorite is the PowerPulse, see
These guys resurrected a ton of batteries that were "dead" in a shipping yard in Kuwait for the military. The Marines are planning on putting solar pulses on every single HMMWV they have after their evaluation testing showed that it helped extend battery life on their vehicles.
Also, read the "Testimonials" section on this page:
There are other pulse chargers on the market as well, so perhaps others have some additional input?