I agree with Sequoia2003. You are probably being lied to (a salesman? No way).
I have both. 5.7L, 4x4, DC, LBed, SR5, Sonar, rear camera, tow pkg (automatically get this with the 5.7 and shouldn't be an extra-cost line item). The sonar and tow package seem to work great together. Make sure your dealer is talking about the true Toyota OEM front/rear sonar and not the GST distributor "trick" of adding an aftermarket rear-only sonar in a non-chrome bumper.
About the only thing I might think of that would cause any "concerns" might be the fact that when towing a trailer the sonar alerts you when you shift into reverse (Duh, there's a trailer back there). I don't consider this a bad thing at all; it's a complete non-issue and if the driver doesn't like the pleasant little beep it can even be turned off with the dash button.
I would suspect the root of your problem stems from the fact your dealer is in Texas and therefore under the thumb of Gulf States Toyota (GST). I live in Texas but cannot buy Toyotas in Texas because of GST forces aftermarket crap, warranties and undesired options down the buyer's throat in an effort to boost their profit margins at your expense. I feel bad for Texas Toyota dealers (er, kind of).
Call a dealer in CA, AZ, CO or NM and you'll probably get a truck with exactly what you want. It's really like there are two Toyota companies from a consumer's standpoint (three actually if you throw in Southeast Toyota).