The blue top is a starting battery, the yellow top is a deep cycle. I think you've got them backwards.
You should use the yellow for your aux battery. Rig a split-charge relay to charge them both when the engine is running, and use the Duo to charge them both off the solar.
Run your aux loads to the aux battery so even if you run it down, you'll still be able to start the truck.
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