I think we need to ask how far our backsides and backs can stand it. I returned to Seattle from a 4,000 mile, six-week trip to AZ and So. CA and aside from being cold, I'm still sore after 10 days. That was driving my AD with camper, by the way.
Tommy, good point! I made the mistake of "not" modernizing the '51's seat while restoring it. Actually made it a bit firmer than stock since I laid 1/2" chicken wire on the springs and then tied each spring to the wire, unitizing things. It's great for the first hour, then NO fun at all.
Recently picked up a couple of 1.5" thick black dense foam pads for longish trips. So far a great improvement.
Our trip miles per day goal had been a max of 400, we may be able to add to that now when needed. But likely not, regardless of miles traveled daily the pads will make for a better day.
RonR