Need more info....
Did it struggle when it had less miles on it ?
What area of the Sierras ? Some are not that steep, nor in altitude
'Small boat'...how much did it weight or how long or what type of boat?
Which Lance inbed camper ? Both my neighbors have BIG Lance campers. One has a F350SD V10 (both the Lance and small utility V-nose trailer with his ATV inside) and goes to The Rubicon many times a years and has NO problems from SF Bayarea up to Tahoe. Other has a CTD with a Lance and has no problems from SF Bayarea up to Tahoe.
Both of theirs single wheel & crew. The F350SD switched to 19.5" and likes it much better than the LT's he took off
The CTD had transmission issues (electrical and told me a common thing) and left him stranded 2 times. Took him about 1.5 years to figure it out....but....it did not struggle going from sea level up to about 8,000 feet
When I go with them and still had my boat, no problem with my 28ft boat and 1996 GMT400 gas 7.4L/4.1 K3500 Suburban and usually lead them...most of the time... :B
Does your V10 have the spitting out spark plug problem fixed ?