Alright, best case scenario your novan is just out of calibration or (and this is what I'd check for first) you've had one of the thermistors go bad, probably the one on the panels.
Now as your working with system with a pressurized potable supply and you just happen to have a SolarSaver tank, I'm going to say more than likely weather the system is drainback or glycol your heat exchanger is inside the storage tank. It's fairly common actually.
It is not uncommon in areas with cold winters for your winter output temps to be significantly lower than summer output, but the fact you can cycle the system manually and get it up to a higher temp really supports the dead thermistor theory (do check it out), but yes you may continue to cycle the system manually as needed to help your backup heater limp along.
But you really should have the system evaluated by someone in person. I or anyone else can give you general advice but without being able to see the system it's quite impossible to spot safety issues or problems that might damage the whole system.
Since you are planning to upgrade the system soon I'm not overly concerned. When you do upgrade be sure your new tankless heater has a temperature sensor for the incoming water. Not all do, infact most just heat the water to a set number of therms above whatever the incoming water temp is. These units are obviously not suitable for a solar preheated systems as you could get scalding water on an erratic basis. This is often an overlooked detail, but there are tankless units designed specifically for pre-heated systems that do a great job.
Hope it helps!
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crashK6