Try and find defensible areas where you only have to cover one or two sides. High ground is also generally a significant advantage, even if it seems inconveniently far from ammo and other supplies...there are several levels I have earned gold on without ever going on an ammo run, accomplishing this by forcing hoards into bottle necks and taking them and specials out with pistols only, and saving shotguns for tank...of which shotguns are absolutely crucial for several reasons - 1. multiplier damage (the closer you are, the more damage you do - at close range, shotguns are by far the most powerful weapons in the game, absolutely vital for bringing down multiple tanks), 2. hoard suppression (no other gun is as effective at dealing with large groups of hoards), 3. high ammo capacity relative to destruction capability (the only other viable weapon considering the massive amount of hoards, the AR, needs to be able to carry 300 extra rounds to last as long, ie instead of 50/360, more like 50/660)
I'll stop by offering a pair of related tips. First is grenades need to be used almost strictly with hoard suppression in mind. In campaign mode and vs., the strategy is to light a tank on fire and let him burn to death. The problem with that strategy is its useless for survival mode as you cannot run away from a tank until it burns to death as hoards never stop coming, and there very well may be more than one tank...thus what you need to do is use your grenades and fire sources to suppress hoards so you can then focus as much of your attention on the tank as possible. The second part to this is not to worry about rationing your supplies (although make sure you're coordinated on their use, throwing 2 pipe bombs at once is a clear waste, the more you worry about it the more likely you'll go without using them and you end up dying with several supplies that go unused.