Seems like the trailers trying to be as lightweight as possible and/or entry level pricing are going to the 20lb tanks. Others are sticking with the 30lb.
don't think furnaces or WH are any more efficient than in the past, but more campgrounds have electric hookup so it does give the option for electric for the water and a small electric heater.
Also, how many people camp for several weeks at a time w/o access to someplace to refill propane, or camp in really cold weather??
Maybe they are responding to changing in camping style as well.
If we dry camp in the early spring or late fall, the furnace is the biggest propane user, and even then a single 30lb tank will last us 7+ nights along with the WH, fridge, stove and oven. so a pair of 20lb would suffice for that.