We have an old 17B and the tanks are smaller than current 17Bs (which are still smaller than 19s and 21s). I believe our black is 7 gal and our grey is 12 gal. We're fine with two humans for 3-4 days that way. We conserve water as much as possible, and it's usually the black tank that's the limiting factor (water in = water out, can't get around it unless you pee in the woods, and then I'm caffeinating frogs). If you haven't camped in a trailer before one thing you may not know is the shower head comes with a real easy toggle, so you just get wet, turn it off, soap up, and then turn the water back on and rinse off. I don't think I've used 5 gal on a shower ever in the trailer!
Whoops forgot to mention: I think you'll be fine with two 20lb propane tanks. We've camped below freezing with the thermostat set to as close as we could get to 60 degrees F (15ish C) and it really sipped at the propane. I know the 21 is a larger box to heat, but I can't imagine it'll be that big of an issue. I agree, bring along the extra tank in case, but I'm pretty sure you won't need it.
Hillary & Jeff
Camping with the sighthound variety pack