Thanks for the replies which encouraged me to dive in this project.
I just finished installing a new American Standard porcelain sink and a really nice brushed nickle Kohler bath faucet (both from Home Depot). The hole for the stock plastic sink I replaced was cut for a "standard" 17" by 20" oval sink and the bath faucet was a "standard" 4" (measured between the hot and cold connections) width. Both fit right in the bath cabinet with no cutting needed.
Tip: based on my past plumbing experience, it will save time and back pain to install the faucet on the sink before installing sink in the cabinet (cuts down on the undersink work). Also caulked the underside of the sink before installing on the cabinet. The hardest part of the install was getting the right quick connection for the plastic (Sea Link brand in my Allegro) pipe to connect to the Kohler faucet which I got from my RV dealer. The new sink and faucet are "bone" color which matches the interior of the bathroom so much nicer than the awful white plastic sink and faucet that were original. The sink and faucet looked so nice that I went back and purchased a new toilet paper dispenser, and two new towel bars, also in brushed nickle. I even ordered a new porcelain toilet to replace the stock plastic one. Finally, I replaced the cheapo shower sprayer with a nicer one from Delta.
All in all, the upgrades have really made the bathroom of my Allegro look as nice and classy as the rest of the motorhome (Tiffin, like many motorhome manufacturers, does not put as much money in the bath area as they do the rest of the motorhome). My wife is thrilled with the new faucet, sink and towel bars and the upgrade cost was reasonable (I did splurge on the Kohler faucet but man, it looks good!).