I had a 1999 272. To winterize I would bypass the water heater and drained it, since I never used it I left it that was all year. I would run the tank empty and then made a pickup adaptor for the water pump and pumped RV antifreeze through the system, should only take 1, maybe 2 gallons tops. With the tank empty I would blow air through the water pump feed into the tank and carefully pour a little antifreeze down that line, never had an issue
On our new-to-us express I did pretty much the same but due to where the pump was I could not easily blow though the pickup line, I used our dewalt leaf blower and covered over the water tank fill and it worked good to push the little remaining water in that line out. I supposed you could use a shop vac too blowing. The air vent to the water tank comes up to the fill neck, similar to the gas tank so if you cover the whole thing the tank should pressurize.