Those vehicles are pretty heavy to begin with, ~17 tonnes for a Mastiff. I should think the downsides of battery weight start to become irrelevant at those levels.
Obviously a move to deployable solar arrays is a big step, but up until the 80s the military led the civvy world in technological development, particularly during conflict, it's only the past 3 decades where consumerism has started to take the lead. The problem is we, well I'm not 'in' any more, so they, only seem to react these days, rather than pro-act*
*I might have just invented that, according to spell checker.