Found it. This detailed (and accurate) plot summary from an IMdB user is bananas:
Plot Summary for "Little House on the Prairie" The Lake Kezia Monster (1979)
Kezia1 doesn't believe in paying property taxes2, and it isn't long before Mrs. Oleson decides to foreclose on the property and purchase it for herself as the family's vacation home3. Nels objects to the purchase, knowing she had acquired it unfairly and that Kezia would have no place to go, but Mrs. Oleson responds by moving herself and Nellie and Willie to the lakeside property. Mrs. Oleson, in a show of pity, hires Kezia to be their servant and forces her to live in the shack. Laura, Albert and Andy watch from afar and are disgusted at how cruelly the three Olesons are treating their friend Kezia, and also aware that Mrs. Oleson used underhanded means to buy the property. One night, after hearing Caroline read a story about monsters, Albert comes up with an idea to concoct a monster to drive the Olesons off the property4. Working with Kezia, the children set their plans in motion, but Mrs. Oleson, Nellie and Willie are too smart for any tricks and harden their resolve to stay on "their" property. Eventually, Laura, Andy and Albert bring out the heavy artillery: creating a Loch Ness monster out of paper maché5. When they hatch their plans, Mrs. Oleson and her children are convinced that the (non-existant) monster poses a real threat to their safety and they beat it. Nels celebrates the success of Laura's plan with Kezia and the others, and it isn't long before Kezia's ownership in her property is restored ... with the promise she will pay her property taxes6, no matter what she thinks.
- You remember the beloved character Kezia, right? No? Yeah, me neither. IMdB credits the character in two other episodes, but they don't rank as memorable to this super-fan.
- Property taxes as a plot point on Little House!
- A vacation home for someone who already lives in rural 1870's Minnesota!!!
- This time, a scheme actually does work because of those meddling teens, but sans their dog.
- Sure. If you're going to go broad, go really broad. The term "Loch Ness monster" wasn't coined until 1933, but hey, they really nailed the period detail with the property tax law stuff.
- A lesson for the ages.