Grade 8 is way stronger than 5, approximately 50%. The important thing is clamping force which is what keeps the bolted joint from slipping. Go to Grade 8 with hardened flat washers both ends and loctite. Torque to 100 ft lbs. Blue Ox and Roadmaster both still use split lockwashers which were proven to be ineffective over 20 years ago. Throw them away, in this case they don't know what they're doing. Remember they're part of the RV industry and you know what that means when it comes to Engineering Excellence.
2007 Damon Daybreak 3060, Ford 16,000# Chassis,
Ford C-Max Hybrid Toad , Suzuki V Strom 1000cc