Hi Kegan,
To determine the height needed for a gooseneck to 5th wheel adapter you'll just need to measure. You'll want to be on level ground and use your trailer's leveling system to ensure that it's fully level and the measure from the ground up to the plate that the king pin is welded to. You'll also need to measure from the ground up to the bed of the truck. The difference in these two measurements is the height needed for the adapter.
The Ranch Hitch Universal 5th-Wheel-to-Gooseneck... view full answer...
It does appear you will want the Gen-Y Hitch 5th Wheel to Gooseneck Pin Box # GY58GR you found on your Dutchman Astoria 2993RLF. I can verify this is the correct pin box if you can give me all the information off the Lippert tag found on the mounting arm which the pin box bolts too. Otherwise you can verify with a few measurements. You said the bolt spacing is 8" but I think it is actually 7-7/8" from front to back on center. The holes will be spaced 2" vertically and the pin box will be... view full answer...
There for a while Hollywood Racks didn't have the trike adapter but it looks like it has come back. What you can use is the Hollywood Racks Sport Rider SE Bike Rack for 2 Electric Bikes # HLY84FR. This bike rack has a capacity of 80 lbs per bike and a max tire width of 5".
You will also need the Trike Adapter for Hollywood Racks Sport Rider SE Bike Racks for Electric Bikes # HLY37RR. This has a max track width of 36" and wheel base of 60" with a max trike weight of 90 lbs. The combined... view full answer...
Since we are not 100% sure of the serial number on your 2015 Keystone Cougar 279RKSWE I recommend verifying the bolt measurements on the pin box. A 1621 pin box is going to have the 5/8" bolt holes spread 2" vertically and 7-7/8" horizontally on center. If you have something different please let me know otherwise the Gen-Y Hitch 5th Wheel to Gooseneck Pin Box # GY58GR is going to be the correct bolt pattern and weight range for your Cougar.
Do remember you will need Safety Chains # GY49GR... view full answer...