We took the same approach for our family of 5 leading up to a 7-wk road trip which we just returned from. I reached the same conclusion you did (rental cost of 7wks/7K miles being more extreme however) - that I'd much rather spend the money depreciating my own RV which I'd have ample time to setup right and use for other trips in the meantime. We bought a 2yo ex-rental (+1 private owner) Coachmen 26QB and enjoyed it for a half dozen smaller trips while getting to know it's nuances, working out proper setup, and making some upgrades. The plan was to sell it after the big trip in favor of a trailer to better fit to our typical needs, but as others have noted we're now in no hurry to make the change.
Here's some of the setup we did (<$500 total) that made a world of difference:
* Tires shouldn't have been an issue yet, but fell under the Michelin recall and were cracking badly this spring, so 6+1 free tires went on before the trip.
* Upgraded converter from WFCO8955 to PD4645 for faster generator charge times ($100 after selling the old one). Most of our charging came from drive time or hookups, but this was priceless when needed.
* Added a Maxxair fan to the BR, allowing sufficient cooling off battery in most situations
* Converted to LED bulbs to conserve battery needed for furnace
* Added WiFi backup camera
* Made custom dash mount for multi-function 7" tablet
* Added 12v/USB receptacle to BR for device charging
* Added 3" foam to BR mattress
* Pop-a-plate/napkin/tissue holders, multiple organizers/bins, etc.
* Oxygenics shower head
* Improved screen door handle
* Remote weather station
Most of these would not have been possible with a rental, and in total, would've made for a much less enjoyable experience managing life in an RV for weeks with 3 young kids (3-8 yrs). We started planning/packing well in advance, so I can't imagine getting a rental and having to do this in a day or two on the clock. Aside from organization and optimizing space, there were little surprises like our receiver bike carrier not fitting past the bumper mount without some bracket modification that wouldn't have been possible to rectify on short notice.
We paid cash to a private seller, and ours came with a 4-yr transferable dealer warranty, but I plan on it being mostly useless after disclaimers except for maybe a major appliance failure (unlikely). The prior owner mentioned the microwave had already been replaced under warranty. Well it "failed" again on the trip, and after 10min of diagnosis I found the AC receptacle had a bad ground due to sloppy assembly, easily rectified.
Of course, YMMV on payoff with a shorter trip, or depending on extra expenses like storage, insurance, financing, etc. But to me this had as much to do with making the trip enjoyable as it did financial sense.