How do I add a subwoofer?

Adding sub properly is a combination of things. You need the following matched together properly. Amp, amp wiring kit, sub and box. The box is the single most important part of the system, it will make or break the performance of the sub. By the sounds of things you are not after a system that will annoy the RV par, but one that will fill in the balance of the sound. Find a practical location for mounting the sub box (it must be securely mounted). To keep it simple and under control as far as the $ goes, keep it to a single 10" or single 12" sub. A prefabricated box should fit into your area that you choose. The sub should ideally be s single voice coil 4 ohm sub with a power rating in RMS of between 100 and 400 watts. Peak power is highly misleading so ensure that both the amp and sub are rated in rms power. Next find yourself a 2 channel amplifier rated between 50w x 2 rms and 100w x 2 rms, this amp is perfect for running a sub like this with ease. For a wiring kit, on the small amp with up to a 30 amp fuse, a 10 gauge wire kit is good, over 30 amp I would look at an 8 gauge kit. You more than likely do not need to go bigger than this.

Go to Wal-Mart of all places to purchase the 8 gauge Scosche amp wiring kit. They are uber cheap there for a really good kit that has all you need for wiring the amplifier properly. For your sub / amp / box, I recommend that you head into a local shop that needs and appreciates your business. They are a shop just like me that offers you this kind of advice, advice that you cannot get from the e-tailer to save a couple of bucks.

Thanks for the info....I actually bought an MTX 8" singe coil sub to replace the factory sub in my '01 Suburban, BUT due to bad info from Crutchfield, it doesn't fit in the factory enclosure.  So I was thinking of trying to wire it up in the RV instead of sending it back.  So if I understand correctly, the receiver needs to have separate outputs for the sub for it to work?

Location depends largely on where I can string the wires.  I'm not looking to shake the entire campground, just add a little more bass than you can get out of a 6 1/2" coax speaker.

Don