A. Well.... on dot/gov tire complaints the Marathons are #1 in ST tire complaints so just about any ST would make a replacement for a Marathon.
If I was going with a ST tire the new Carlisle RH or HD are popular. The Provider has been getting good reviews on all type of trailer websites.
Maxxis ST8008 seems popular on some rv websites.
Some folks have gone to a LT235/75-15 C load range or a Goodyear Wrangler HT in a LT215/85-15 D load range both at around 1985-2085 lb capacity.
B. the 205/75 and the 225/75 both use a 6" wide wheel........the 225/75 is 1" larger diameter.
Changing tire size/diameter means checking out clearance issue's for your particular trailer.
"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers
'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides