Air Tabs have mixed RV owner reviews - some notice modest stability improvement, others feel no difference. Better investments for handling are sway bars, steering stabilizers, and proper weight distribution.
The front leaf springs on 2017-2021 F53 chassis are identical in configuration and mounting points. Your Air Lift kit from the 2017 Winnebago should work on the 2021 Holiday Rambler Invicta since both use the same F53 chassis specifications.
Transmission issues - check fluid level and condition first. Regular fluid changes prevent most problems. Scan for codes to help diagnose.
Excellent maintenance approach on your 2016 F53 chassis at 87,000 miles. Your decision to replace the original shocks with Bilsteins and service the Dana Spicer 17060S differential was spot-on timing.