We had the OEM Marathons on our 2005 Safari 25FB. We switched right over to the Maxxis load range E's without any problem. We ran our Maxxis at 80 psi. Our first set of Maxxis went 24,000 miles before experiencing tread separation. We replaced these with another set of the Maxxis. They did not do as well. They only went about 13,000 miles before the tread separation began.
The OEM cast wheels handled the Maxxis fine. Being that you have a 2005 model year, you probably have those same wheels.
Here is a picture of those wheels.
SuEllyn & Brian McCabe
WBCCI #3628 -- AIR #14872 -- TAC #FL-7
2015 FC 25' FB (Lucy) with HAHA
2005 Suburban 2500 Quadrasteer (Olivia) & 2011 Silverado 3500 (Fred) with Outfitter Truck Camper (Ethel)