So, we received and unpacked our three new 2021 Menards Tanks yesterday--Sinclair, Texaco, and Pepsi.
The decks are, as Cabinet Bob said, 1/8 or 1/4 of an inch higher than the problematic Lifesaver cars. The trucks are much simpler to access and service, making it easier to treat any bent axles. However, of the 3 cars, the only even slightly defective axle was right over where the shipping box had been punctured--understandable, then, that that axle was slightly bent. The other 11 axles (and 23 of the 24 wheels) spun straight and true, without any noticeable wobble, as I recall it.
The springs in the couplers are solid. The uncoupling pin now seems flat instead of curved--this is an improvement, as it will get hung up on track irregularities less often (hopefully not at all!) compared to the older Menards cars.
The wheels remain filled.
I eyeballed them side-by-side with a Menards Australian Gold Classic Tank car, Menards Lifesaver Modern Tank, and an MTH RK CSX Blue & Yellow Modern Tank (30-4224-1C). Pictures might be attached below.
I ran a train consisting of K-line NYC S-2, MTH Modern, Sinclair, Texaco (or Pepsi), Pepsi (or Texaco), Australian Gold, and then Lifesavers around our layout. This layout is made of tubular Lionel & Menards 3-rail track with several 022 Lionel switches and one AF O-40/42 switch (I'm not sure what radius the AF switch is). I was hoping for at least 10x around, but after maybe 8X around or more, I increased the speed to 12.5v.
On one of the next S-curves, the engine fell over sideways, causing the train to (hopefully understandably)...stop abruptly. The MTH tank car derailed but the others did not, as I recall. (Maybe the front of the Sinclair derailed?)
Regarding weight, I weighed all of these cars on a kitchen scale.
The MTH car was 12.5 oz
Lifesavers: 9-9.5 oz
Australian Gold 9 oz
2021 Pepsi, Sinclair, Texaco: 7 oz each.
I did not notice any of the paint flaws that were on the Lifesavers last year.
Gripes: the lids do not open, unlike the Menards Classic Tank cars (Australian Gold, Frisco etc.) This means that you cannot easily add weights, if desired. Also, it looks nowhere near as cool. 
TL;DR: I was very impressed. If, as a result of the $25 price, all of the new Menards cars have similar improvements to QC and have working trucks, the price increase was well worth it.
Pictures:
01: 3 Tanks--Menards 2020, 2021, MTH (top to bottom)
02: 3 Tanks--MTH (L), Menards 2021 (C) Menards 2020 (R)
03: Menards 2020 flanked by 2021s
04: MTH CSX & the other Menards tank cars
05: Menards Classic, 2021, MTH, Menards 2021, 2020, 2021
06: Menards Sinclair 2021 truck
07: Menards 2021 Pepsi detail
08: Menards 2021 Pepsi detail 02
09: Menards 2021 Sinclair detail
10: Menards 2021 Texaco detail
11: 2020 Lifesavers -- Nothing like this paint flaw this year!
