It will work at lower power. I've drained my aux battery to totally dead, while my Antec 120mm fan was turned on, a whole bunch of times. No problem for the fan.
Not sure if over-voltage would hurt it, but I would assume it was more likely to than under-voltage. You can limit the voltage with an LM7812 voltage regulator (limiter):
Cost about a buck and a half at an electronics supply store. DON'T forget to use a decent sized heat sink for the thing. (A 2" piece cut out of any ol' aluminum channel works well.)
As for the solar...I dunno about that. I have my doubts as to how many watts a 5w or 10w PV will actually produce. Personally, I'd say it was one of those things that just has to be tried to be sure.
Don't forget to tell the rest of us if it works or not.
Current: 76 E-250, bubble-top, self-contained|couple of old Yamaha enduros
Previous wheelers: 41 Willys|78 FJ40|78 Bronco|84 Bronco|74 Ramcharger|78 Ramcharger|79 D150 PowerWagon|77 D100|79 D400 dually, converted to 4WD, utility bed, 10' Lance|75 Westy|69 Scout, RHD|bunch of others|bunch of bikes|couple of boats|couple of motorhomes|blah blah|so what|not my idea|just doin' what I'm told|wank wank|this space for rent|candy is dandy|but liquor is quicker