I think you are referring to the trailer tires when you ask what type? I can't offer much advice there as I just use regular trailer tires - I did put new ones on last year though, and bought a load range higher than I really need. Believe they are LR E.
I did that because the weight of my trailer weas getting up near the top of the range for
what I previously had on so I felt it might buy me a little extra margin of safety.
I did want to replace the Marathons that came with the trailer because I heard so many horror stories - although mine were still fine when I changed them.
Bought the new ones at Jackson Centre and I thin they are called Towmaster or something similar.
As for when to change tires, many sources recommend about 5 years and that is what I do.
On a previous trailer I had tires fly apart (They were Marathons!) but at that time I wasn't aware of the age factor - I just felt that if the tires had tread and no sidewall cracks I was good to go.
I now know better!
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.