I have 32 foot travel trailer, with a ducted ac unit in the front (15,000 btu)that dosen't cool the hold trailer... the trailer is already wired for a rear ac unit, and my question is should I buy another 15,000 btu unit, or a 13,500 btu for the rear...also should I buy the AC from my local dealer, or buy one on line, and have it installed by a RV shop? thanks for your help
Only you can properly answer these questions.
If the difference between the 13.5k and 15k btu units are within what you are prepared to spend then you should probably get the larger one. I would think, though, that just having the smaller second one should be enough given that you only habe one now and from the sounds of your post it does not sound like Death Valley inside the RV.
Dealer or no? Again, call around and see what they will do for you - supply and install vs install your purchased unit. That's all there is to it. I would also ask if they have any preference for units and then let that factor into your online shopping. You don't want to buy something they are not willing to or comfortable installing and then find out the price changed as a result.
Both Coleman and Dometic units install in a 14" square cutout, and sit on a gasket that is compressed with 4 bolts. I would make my own choice on what to buy. The dealer will try to sell you either what he has in stock, or will make the most markup from. If he did not want to install my choice of units, I would go to another dealer.