"Apparently nobody agrees with him that the smaller unit is actually preferable? " I think you just answered your own question!
(fyi, I switched out our 13.5 for a 15k in our current 35 foot TT. BIG difference in cold out-put, and NEVER had a freeze up). The bigger unit will run better, put out cold better, will cycle on-and-off which will give time for any ice build-up to melt. The 13.5 will have to work and work and work and work. IT'S the one that will be more likely to freeze up, as the compressor will never shut off. It will constantly run causing the coils to freeze because its trying to work so hard to keep up. This is one situation where bigger REALLY is better!