I've used the RCM procedure ever since getting the Classic in 2004.
(RCM= reliability centered maintenance)
After discovering just what a PITA it was to empty the closet to gain access to the pump, and getting severely scolded for not returning all the DW's "essentials" to their previous locations, this seemed like the logical solution.
I started by using 4gal but soon realized that just 2 in the tank would do the trick. I buy 3 every fall using the 3rd to pour down the drains to keep the traps full and some extra for the crapper bowl, checking several times during the Winter to make sure there is some there to keep the seals from drying out.
One minor down-side….You will spend a little more time flushing the system in the Spring. Not a serious concern for us, FW tank not used for drinking and I sanitize with bleach at Spring get-ready. Clearing the lines with compressed air speeds the process.
The RCM for Winterizing….
1) Drain FW tank, close valve.
2) Drain hot water tank and close by-pass valve.
3) Open all three low water line drains. Raise and lower tongue to assist draining.
4) Attach air compressor line to the inlet fitting for external water supply, level trailer.
5) At 60psi, clear each low water drain one at a time, keeping one line open at all times, do NOT close all lines at once with compressor connected.
6) Inside…Place shower hose in tub,open one faucet, (close LW drains), hot & cold till clear. Same for kitchen,(don't forget toilet & kitchen sprayer), and BR sink.
7) Remove compressor line, close all faucets and add antifreeze to FW tank.
8) Turn on WP, wait 'til pump stops.
9) Turn on each faucet, hot & cold 'til running nice and pink, remove kitchen faucet filter.
10) Pour a little AF in toilet.
11) Leave low water drains open