I get the 2 1/2 gallon jugs from Pepboys, Walmart, etc. They come with a hose cap that fits the fill opening nicely. My 2012 Solera uses about one gallon every 1700 to 2000 miles. I add based on mileage and try to avoid having the warning light come on. On our MH the warning light comes on at about 4300 miles from full DEF. Won't quite take 2 1/2 gallons at that point.
2012 Solera "S"
Calif SF Bay Area