Is the tank discharge fitting at the bottom of the tank, or is it on the top of the tank with a pickup tube inside the tank? If on the top with a pickup tube, what's the tube material--pvc or metal?
I should have read further before I posted...your problem has to be a pickup tube that urine has eater holes in...in fact, the bottom two thirds may even have fallen off....and your aluminum tank isn't far behind in being turned into a colander by urine.
The simple temporary solution: a piece of 1" hose through what's left of the pickup tube, plus 1"-1.5" adapter to connect the 1.5" hose needed to connect to your Dometic discharge pump (you haven't mentioned a VacuFlush, but even if you have a VacuFlush toilet it wouldn't have anything to do with your holding tank unless it's one of those gawdawful "sailvac" tanks
that does double duty as both a vacuum tank a holding tank). Wrap the fitting in the top of the tank with lots of duct tape or whatever you have handy to prevent odor
from leaking around the hose.
When you get back to the US, you'll want to replace the tank...and plastic is the ONLY material to use for waste holding. Ronco Plastics Ronco Plastics Marine Catalog
is your best source. They're in Tustin CA...build TOP quality tanks
for a very reasonable price
and have more than 400 shapes and sizes, over 100 of which are non rectangular...and they install fittings--including pickup tubes--in the sizes and locations specified by the customer when they make the tank.