If by stock you mean without the DLC, there isn't really any good area. A good one I can think of is in the Rust Commons West, at the top area of the map (including Tannis' area) you can find about 5 red chests and 1 white chest. They won't scale if you aren't on playthrough 2.5, however. If you DO have the DLC, the armory is the clear choice. Happy hunting!
EDIT: Depending on your run-through(i.e 1/2/2.5) The Lost Cave where you complete the shock crystal Harvest mission has the most, though define 'midlevel'. Going into the mine, you encounter two red chests, both atop rises on either side of the path. One is next to a save pole, the other next to all the Skagg Holes. Going further in, a red chest is in the dead-end cave to the left of the first bandit battle. A second red is next to the large metal 'hut' in the middle of that first encampment. Cotinuing in, past the broken mine rail line and across the bridge, are two more red chests in an area inhabited by a large number(8-10) of Alpha Skaggs, though only two at a time come out.
EDIT: The Rust Commons East and West both have 9 red chests each (as well as some white chests) and both maps aren't exactly small though.
EDIT: For sheer volume with liitle or no opposition, try Claptrap DLC4 (Tartarus Station and adjacent areas). Drmaxman 21:32, May 10, 2012 (UTC)
EDIT The Crimson Fastness has a good selection of red chests (5). More importantly, it is a small area, and can be run very fast. The opposition is lance, so running it remains fun, gets you exp/proficiency, and is not too hard. With a specialized build/layout, you can easily run it in about 4-6 minutes. It is not THE area with the most chests in vanilla Borderlands, but I think it is arguably the best place to make loot-runs. happypal (talk • contribs) 12:50, May 11, 2012 (UTC)