Everything to do with engines, options, upgrades.
I replaced the COPs (ignition coils) and took it up a long steep hill. It was only able to do 72 mph last time with the missing, now it was at 80 and accelerating up the hill. Much, much better! Thanks to everyone for their help.
I think it is just a coincidence, since speed of engine does not work like this, but 72 vs 80 is exactly 9 of 10 (as 9 working cylinders in a V10 engine).
2000 Concourse dinette, on 2000 6.8L Ford E350 Triton V-10 Chassis (built in 1999)
I took the RV up to Santa Fe this week. It ran perfect up the steep climbs! So my problem ended up being the fuel pump and the ignition coils.