If I understand you correctly your truck has a 30 amp fuse however the manual/specs. call for it to be a 40 amp fuse? Are you the original owner of the truck?
If the fuse has been changed from the factory spec of 40 amp to a 30 amp I would certainly change it back to a 40 amp.
19'Duramax w/hips,12'Open Range,Titan Disc Brake
BD3,RV safepower,22" Blackstone
Ox Bedsaver,RV760 w/BC20,Glow Steps, Enduraplas25,Pedego
BakFlip,RVLock,5500 Onan LP,Prog.50A surge,Hughes autoformer
Porta Bote 8.0 Nissan,Sailun S637
Correct Trax,Splendide