Kiwi,
Why do you need to jack the airplane up, if you just have a flat tire?
There are lots of little battery operated pumps you can plug into a cigarette lighter adapter (or some other electrical power source). These will easily pump up a flat tire in a short time. They'll take a tiny bit of power from your battery, but not bad.
Or, you could carry a good bicycle type pump and pump up the tires. They are not that large.
I presume that these are tube type tires, so this should be easy. I sure wouldn't jack an airplane up and remove the wheels to get them aired up if I could avoid it. There is always risk in jacking an airplane.
Finally, if you have SLOW leaks, try filling the tires with nitrogen.
But, if your tires are going flat overnight for no apparent reason, I'd bet that you've got a pinched tube or a bad valve stem, and if that's the case, you want to REPLACE that tube. Point is, if it turns loose on you while landing or taking off, it could create some serious havoc.
In other words, find out what's wrong and get it fixed, especially if its a recurring problem. Most tubes will lose some pressure over a period of time, and we need to monitor them. But if your tires are going flat overnight, get em fixed.
MTV