If you do decide to fill it with the bed up, two things to remember;
1. Be sure to block it VERY securely before going under there. Guys have been killed doing that.
2. Leave the plug out the first time you let the bed back down. That way, if you've overfilled it, the excess fluid can run out. Otherwise, you could blow the seals out of the hoist.
It's much safer to fill it with the bed down if possible. It's a little harder to get at the plug, but it can be done. I used a suction gun to squirt the fluid into the hole.
