... Now, checking the GD parts website and selecting my year and model (2015 Reflection 313RTLS), actuator is shown as 170003 : ACTUATOR-40" W/ HIGH SPEED MOTOR 28:1 VENTURE.
So ... who do I trust, my own TT or GD? Do I replace my broken part with the same part I have now (18:1), or do I go with the manufacturer's parts website which says 28:1 motor (which seems to be Lippert part # 136373)?
Yeah, why couldn't our gearbox have the gear ratio labeled like some I've seen for sale. I'll have to open mine & count gears to know the ratio.
I was working on my RH slide yesterday on the RV dealer's half baked fix for slide actuator stop can hitting cross shaft, this was done for the 1st owners.
I should have pulled the motor/gearbox down & opened it. Since I'm still working on it I will. If it's 18:1 that's what I'd replace it with.
edit: Opened my RH slide gearbox, it's 18:1 per having 25 teeth on 2nd white gear and 53 teeth on the clutch gear.

Venture Data Sheet
http://www.venturemfgco.com/uploadfiles/image/file/RVSO-Techincal-Data.pdf shows:
18:1 gearbox has 1200 lb clutch
28:1 gearbox has 1500 lb clutch
Options:
1 - Go with GDRV Parts, 28:1 is listed for my Reflection also.
2 - Call Venture Mfg with SN of the gearbox to get their answer.
Remembered this thread from last year, that's what he did:
https://www.granddesignowners.com/forum/showthread.php/12230-Slide-Out-Motor-Repair
3 - Count the gears to figure the gear ratio - 191072 18:1 lists as having 53 tooth clutch assembly, 191073 28:1 lists as having 61 tooth clutch assembly
Also noted that the 18:1 & 28:1 inner gears are different sizes 2-1/4" inner gear, 2.5" inner gear -
http://approducts.net/catalog/index.php?main_page=index&cPath=48_59
On these ratio calcs I'm just going off the pictures for these gears sets, so I may not have the tooth count right.
Your photo looks to have 25 teeth on the white plastic gear.
I had to go read how to do this for a gear train -
https://woodgears.ca/gear/ratio.html https://www.wikihow.com/Determine-Gear-Ratio
18:1 ratio = 8.5 x 2.12 = 18.02:1
1st gear train: driver gear on the motor shaft is 10 teeth, 85 teeth on the driven gear (per JBarca's thread), ratio 85/10 = 8.5:1
2nd gear train: driver gear 25 teeth, driven clutch gear 53 teeth, ratio 53/25 = 2.12:1
28:1 ratio = 8.5 x 3.21 = 28.8:1 so in the 28:1 ballpark.
1st gear train: driver gear on the motor shaft is 10 teeth, 85 teeth on the driven gear, ratio 85/10 = 8.5:1
2nd gear train: driver gear 18 teeth, driven clutch gear 61 teeth, ratio 61/18 = 3.21:1
Lippert And Venture 18:1 Motor Replacement Gear Set 191072
https://pdxrvwholesale.com/products/venture-181-motor-replacement-gear-set-191072
(1) EA A-8504-01 18:1 2nd Reduction Gear
(1) EA 8579-11 18:1 Clutch Assembly (53 tooth)
Lippert And Venture 28:1 Motor Replacement Gear Set 191073
https://pdxrvwholesale.com/products?keywords=venture+28:1+motor+replacement+gear+set
(1) EA A-8504-02 28:1 2nd Reduction Gear
(1) EA 8579-11V 28:1 Clutch Assembly (61 tooth)