Plasmatronics still in business?

I wouldn't be so quick to suspect the PL20 is faulty, the voltage difference could be a crook connection like a fuse cct breaker, wire or just a plain connection fault.
I had the same with a system which includes a PL40 and there were two connection points that individually dropped he voltage by a minute amount but both added up to a couple of volts.
Might be time to go right through all connections including spade fuses (a known fault point) and replace/clean/reterminate everything especially after 7 years. Don't forget the ground/earth connections.
MPPT are not the best for many RV setups either unless you are running higher voltage panels etc.