You know I wouldn't recommend it and I have done this.
Isn't there a load capacity weight rating in actual pounds? I believe a "C" rating is somthing like 1880. With all "C" rated tires that would give you a capacity of 7500+ pounds at max tire pressure. If your fully loaded rig is 6000#'s or less, I would stick with "C" all the way around.
If the above fits, "D" rated tires will be more expensive, provide a rougher ride and not offer any more benefits. If over 6000 #'s fully loaded then my suggestion is it is OK to go to "D" rated tire.
1966 Mercury Park Lane 4 DR Breezeway 410 4V, C-6, 2.80 - Streamless.
1966 Lincoln 4 door Convertible 462 4V 1971 Ford LTD Convertible 429 4V Phoenix ~ Yeah it's hot however it's a dry heat!