Adding small amplifier to existing TH stereo

Hey guys, I am wanting to add a very small conservative and efficient amplifier to my existing Sony head unit in my '08 WW FS2500 Superlite TH to get a better clean sound from the two outside speakers when turned up around the bonfire! We dry camp 100% of the time out in the dunes or deserts so having the stereo turned up a little is not an issue as it would be in a campground. I understand adding an amp will use more of my 2 rv batteries power, but I have changed out most of my lights to LED's so that should help a little. I was thinking of adding a GOOD QUALITY 25W RMS per channel amp. My TH has a speaker selector switch to switch the speakers from inside front to rear or outside or both. My question is the way the head unit is wired to the selector switch, should I get a 2-channel amp, or a 4-channel amp? I would like to keep the speaker selector switch if possible. My main thing is too get the clean RMS power from the amp to the two outside speakers. Thanks!

p.s. The amp I am considering is a KICKER 25 watts RMS X 4 amp that only draws 30 amps of power from the rv batteries.