Great questions and I have all the answers for you... I have a RTV900R and own a regular 5x10 landscape type trailer & a Performance (Brand) 5x10' trailer with a slanted front rail. I also put a 1 5/8" lift kit on my RTV and Essex 27" 589 tread tires. It loads great on both trailers. I also loaded it on both trailers before the lift kit and big tires. I settled on the Performance trailer for the RTV because I got a little more length with the slanted front, this gets the engine and trans of the RTV further forward to help with the tongue weight. It's also hot dipped galvanized which will keep it from rusting much longer than a painted trailer. I also use the trailer to pull behind the RTV to haul hay, tree limbs & other bulky things.
Go for the 5x10 it will work fine.
As for RTV v.s. Mule... It's not a question... RTV. I had mine loaded down with horse feed in the bed 500#, two 250# passengers, pulling the 1000# 5x10 trailer which was loaded with 900# skid steer grapple & 450# back hoe attachment. It pulled this just fine, including up and down two very steep hills. As an example when driving my Ford SD up the one hill, it's so steep when you get to the top you can't even see if you are on the road because of the hood of the truck. My RTV just pulls and pulls. Yesterday I found a rock climbing Jeep stuck down a very steep hill and assisted in getting him out. After he got out I drove down the hill to where he was stuck and attempted to back up the hill. It was so steep the RTV would not even spin the tires. I then did a three point turn and faced the RTV up hill and put it in LOW and pulled right up with out even spinning or struggling. Jeep was really humbled.......
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