Are you interested in just adding a receiver to your current set-up or do you intend to replace/upgrade the speakers, too?
If you just want to add a receiver, it is important that you match the impedance (ie, resistance) of the speakers to the impedance the receiver is designed to drive (note: the receiver doesn't have an output impedance; it is the speaker that has the impedance. The impedance you may see listed for a receiver refers to the optimum speaker impedance that it is designed to drive). The impedance value is reported in ohms.
Generally a lower impedance speaker will draw more current (because there’s less resistance) and you could blow the receiver’s output transistors (due to the increased heat) if you crank up the volume (eg, speaker rated at 4 ohms and receiver rated at 8 ohms). Again, you are looking to match the impedance of the speakers to the rated impedance of the receiver.
I don’t know what the speaker specs are for your TT. In my 2006 Jayco Eagle 264 BHS, I have speakers that are rated at 6 ohms and have a max power rating of 90 watts (Magnadyne LS3CMW-JCO 3” ceiling mounted speakers). The OEM Memorex 5.1 receiver died last year. After months of searching, I decided to replace it with a Yamaha BRX-610 receiver for the following reasons (which I picked-up at about 30% off list price since it was a return item):
(1) it is a 5.1 channel receiver;
(2) max output is 80 watts/channel rated at 6 ohms (which matches up with my OEM speakers);
(3) has a built-in bluray player, so one less piece of equipment I need to account for;
(4) has a height of only 3-1/8” which can be install in my entertainment center (a regular receiver would not fit in the narrow slot as designed by Jayco);
(5) has FM radio (but not AM (?)) and can be connected to the Jayco antenna;
(6) is 1080p-compatible HDMI (I purchased a Westinghouse 40” LED with 1080p on sale at Best Buy); and finally,
(7) has a USB connection for iPod/iPhone.
It’s also supposed to support Netflix over the Internet, but I haven’t tried that feature because I really don’t intend to watch much TV when camping (some users/reviewers have indicated that the receiver doesn’t work too well with Netflix).
In any event, I hope this helps.