We picked up the 250 gallon plastic tank with cage nearby for $150.00 that's being used for black water, that has a sewer service, cost to empty is $90.00 which is much less than trying to service 250+ gallons.
For $150.00 included a 50 gallon tank with lid and a 65 gallon, no lid has opening at top with plastic screw.
Can't remember exactly but a ~500 gallon would cost significantly more to service for sewer than a 250 gallon.
In another forum a person said he used food grade 55 gallon plastic drums to build a float. Said he paid from %5.00 - $10.00 found on craigslist for the plastic drums.
Locally the least expensive 55 plastic drums found are $15.00 each.
Approx. 125 to 150 miles away.
Other drum prices craigslist varying distances in other towns and cities from #20.00 to $25.00+ each.
55 gallon drums sounds like a good alternative if inexpensive enough and not too far way.
6 x 55 = 330 gallons.
Cost for 6 x 55 gallon drums at $15.00 each = $90.00
250 to 300 miles away RT fuel cost to pickup $55.00 to $65.00.
However I'm uncertain the cost for sewer service i.e. if the sewer service goes by the amount sucked out of a tank or the tank size. If amount would likely only need 4 x 55 gallon drums.