Here is part of my email exchange with the Caster City sales rep:
Subject: Re: Casters for a 1200 pound pop-up camper dolly used outdoors
Craig:
We always recommend that you use the largest wheel possible, but I wouldn't recommend anything smaller than a 6" diameter wheel.
I would not recommend the injection molded Polyurethane Tire on a Plastic Center--you will need to replace the wheels more often than you will want.
Even though the center can rust, the Polyurethane Tires that are chemically bonded(glued) to an iron center will offer much better service than the injection molded style. I am partial to our Slightly Crowned Polyurethane Tire on Heavy Iron Center.
The slight crown on the tire makes it a little easier to maneuver/change direction than a wheel with a flat profile tread face, plus they have Sealed Precision Ball Bearings that require no maintenance.
http://www.castercity.com/cm9.htm
https://shop.castercity.com/ProductDetails.asp?ProductCode=9PYC6x2-S
https://shop.castercity.com/ProductDetails.asp?ProductCode=9PYC8x2-S
Any of the Polyurethane Tires on METAL Centers would be a good choice and you may want to consider using either the Welded on Hand Operated or Bolt On Foot Operated Swivel Lock for two of the casters.
The swivel lock will give you better control if you have to roll the dolly more than just a couple of feet--when engaged the swivel lock makes the caster function like a rigid caster, but if you need to "jockey" it into the storage position, you can disengage it and you have 4 swivel casters again.
Once you determine which caster you would like to use, contact me and I will be able to save you at least 25% off the prices on the website.
It might be easier to discuss all of the options on the phone, so contact me when you can be near a computer if you would like to discuss your application.
Thanks
Greg Howard
Caster City, Inc.
8635 Bright Angel Way
Las Vegas, Nevada 89149
Phone: (800) 501-3808 ext311
7:30AM to Noon, and 1:00 PM to 2:30 PM, PST
Email to: Sales@CasterCity.com
Fax: (702) 522-1904
Website: www.CasterCity.com