The MORryde is an excellent pin box - that's what's coming on the new Solitudes and Momentums. The pin box will not affect your frame warranty since that is the pin box Grand Design is installing on their units, anyway. As you stated, the GVWR of the Reflection 337 is 13,995 lbs. I'd call MORryde and have a conversation with them about which model since your RV is right on the borderline. MORryde is most helpful. I think the model up to 14K will do fine and will likely give you a better ride. The higher rated one might be too stiff. I love our MORryde pin box (and loved our Reflection 337 which we were full time in for 4-1/2 years).
Rob