+2 on the Sta-Bil.
If you can't or don't want to remove the battery, a battery tender is your best bet. That big website named after a river in South America has a huge selection of maintainers, including name brand units starting around $15 (Stanley and Black & Decker).
I have various B&D, Schumacher, and assorted ultra-cheap Chinese units. My favorite is probably the 1 AMP Schumacher SP2 for the extra long cords, solid construction, and relatively small body with no corners to scratch paint or upholstery. Not sure that one is still available, though. The 1.5 AMP SC1319 is readily available, seems well constructed and works fine, but it's boxy and clunky...Schumacher is an old-school battery charger company and this unit is styled like a traditional battery charger, the kind your father and grandfather used to have.
The B&D BC2BDW is a close second. Higher output (2 AMP), well made, with corner "bumpers", and it comes with a cigarette lighter adapter, which everybody else spanks you about $15 for. The downsides are shorter cables, and the unit is a little too heavy to leave it "hanging" in a pinch. Great unit otherwise. For a little bit more money, you can get the BC6BDW, which is the same design but puts out up to 6 AMP, so if you ever need a (relatively) quick charge, it can do it much faster than your typical battery maintainer. The 1 AMP BM3B is dirt cheap (less than $20), works, and has decently heavy cables and connectors, but is lightly built and I wouldn't trust it for months at a time...I've seen the insides and the circuitry seems cheap and flimsy...I'd be worried about coming home to a burnt-out shell.
Other than my own personal preferences, all but the no-name Chinese units do the job just fine.
If you don't have power available or don't want to leave a charger on your truck, consider investing in AlfaOBD and a cheap Bluetooth OBD II adapter. I've never used it on my RAM, but I know that on my '14 Charger R/T, I can use it to change my car's configuration to "Shipping Mode". As I understand it, that's roughly the equivalent of taking the battery out. I don't know that it'll keep six months that way, in fact, I'm pretty sure that's not the intention. Still, it's an option you might want to consider if you can't or won't take the battery out or run a maintainer, and don't have someone available to start it up every couple of weeks.
Good luck!