I used to keep my jack down with the weight off of the tires during the winter storage, 4 to 6 months. Never had a problem. Then one spring on my first camping trip I blew a seal. So I assume it could of been caused from keeping weight on the jacks for such a long time or it could be that the seal was 12 years old, go figure. The seal that blew was internal to one jack. FYI, Cheaper to buy a rebuilt jack then to have your jack rebuilt.
Hollis I added 20% to the total weight of my motor home. Divided the total by 4 then divided that number by 2. Which, for me, just comes under 4 tons. Not sure if this is the correct way or not but seems to be working.