If the gate valve outlet is just a piece of pipe that has been cut off then you still have some options? You drain the tank and cut the old useless valve off cause it needs to be replaced (18$ at the rv place here) and get a glue cap for 3-5$, a rubber test cap for 3-5$ and re-use it over and over, or buy a roll of good duct tape and dry/clean the pipe off on the outside good, cover the end of the pipe with several pieces layered that fold down/up the end of the pipe by 3-4"'s and then wrap around the end of the pipe to secure the tape folded over the end of the pipe. Not re-usable but workable and the rest of the roll is there for other things? Or you can cap/plug the other opening's in the tank, buy an elbow and piece of pipe to glue onto the gate valve end and do a real pressure test on the tank itself?
The higher off of the ground the tank is the higher the pressure is going to be.
For non pressurized (not bragging but I put 80-psi on mine before install but I had the resources at the time) for dead water weight as I call it for every foot above the ground/gravity level there is a 2.31 PSI pressure increase.
The pipe and elbow I mentioned to glue onto the gate? If you turn an elbow up with a 10' piece of pipe and fill it up to the top then your water pressure will be roughly 20-22 PSI.
As explained to me quite awhile ago?
If you have a 1/2" pipe stuck up 100' in the air with a 20 gallon tank and right beside it you had a 24" round pipe stuck up in the air 100' with a500,000 gallon tank on it? Which one would have the most pressure?
They will both have the same pressure because they are both at the same elevation above the ground/gravity line? Thought it was useless knowledge stuck in my head with my knew job and maybe it helped but now it's knowledge useless or not for your head?