I don't understand the mechanic saying it would take 25 gallons??? With the MH level and the water tank empty, we do ours with three gallons, might be able to do it with less. All you need in the tank is enough for the water pump pick up line to pick it up and pump it through the water lines. 25 gal would be a half full tank.
To get it into the tank, I bought one of those little pumps for $11 that you put on a drill. Then two short pieces of plastic tubing and you can pump it from the gallon jugs into the water tank.
If you have them, the trickier parts are getting the antifreeze through the ice maker and all of the lines on the washer dryer. Running the pumped antifreeze through all of the water lines (don't forget the outside shower faucet) will get the antifreeze to the gray tank, then put some in the black tank and the water traps and you're done.
The first time we winterized it took us about 4 hours to figure it all out. After, less than an hour.
Bill