I found that the reason my pump was a ball of grime was it was the source of the leak. You might consider cleaning it up, breaking it down, and resealing the pump. This accomplishes a couple things. First, you ge a good seal on the pump and a good eye on the guts to see if anything needs replacing. Second, you can get all the old nasty fluid (mine looked pretty gross, lots of contamination) out and all new fluid in.
Regardless, I found that you can get to the plug with the bed down just fine. If it's the square pipe plug, a big crescent is perfect. I pulled it off, and as mentioned above, used a pump & hose setup used for gear oil to fill it. Set your new fluid bottle on a 5-gal bucket or some such and pump away. I used almost two gallons to fill an empty system.
I ran it up & down a couple times and it went to full extension the 1st time and every time after, AND, I dont leave a puddle under the pump anymore.


1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion!
:P