Your Spartan's air bags are connected to a height sensing valve. This 3 way valve is operated by a link that is attached to the frame. In a midway position it will block off the air in your air bag, letting none in or out. If you loiad the coach up, it will settle due to the extra weight. As the frame moves down it actuates the arm on this valve and connects the air bag to the air tanks so that the compressed air can pump the bag up so that the coach regains it's set ride height.
By the same token, if your coach starts to ride high after losing some weight (that's everyone's goal isn't it
) then the arm operates the valve in the opposite direction, allowing the air bag to drain the excess air and vent it to atmosphere until the coach settles in at it's designated ride height once again.
If your bags are filled to the prower amount when you park they should hold their pressure for a reasonable time unless there is a leak. The leak could be a hose fitting between the bag and the valve or it could be an internal leak in the valve. Note that if you are parked on a slight incline the bags will drain anyway because of the weight transfer to one end of the coach.
As to taking forever to fill them up, if you run the engine at a fast idle this will improve. Empty bags do take quite a while to fill if the engine is idling. Probably not 10 minutes though. If you rev the engine to 1500 RPM you should be able to fill a complete set of dumped air bags in less than a minute. If that's not happening, you have an issue in delivering air to the air bag. Assuming that your filter-drier is clean and you are capable of pumping air as designed, I'd say that the issue most likely exists in the ride height sensing valves because they are draining them out when they should be holding air in and they are also not allowing air to fill them very fast. Note that in this case you will see good air pressure on ther dash gauges but the air just isn't getting into the bags to raise the coach. If you have a problem in putting out enough air in the first place your gauges will indicate lower pressure. The only other possibility is if you are making enough air but it's leaking out too fast to raise the gauges. But that you would hear so I don't see that as an issue.
It sounds to me like it might be time to have a Spartan service center take a look at them.