??Time for new shocks??

I replaced the OEM Rancho shocks on my 2012 F350, long bed, King Ranch, in Feb, 2013 with Bilsteiln 5100's; 8 years and 65000 miles ago. 60k of that 65k miles has been pulling/towing an 18,000 pound fifth wheel (no off road). I also replaced the tires with the next larger size and added Air Lift air bags with onboard compressor. We just completed a 4k mile trip with the five, and the truck seemed to hit hard on every bump. Being disabled, it literally beat me to death. I've talked with Bilstein Tech Spt about the life of the shocks, and can't get a definitive answer. Is it time for new shocks? I don't mind the expense, but don't want to spend ~$500 if it's not needed. I don't see a lot of traffic on this forum, but I really need an answer.