SmithCrazy3,
I don't have a big rig but most of the Premium loops will work for your DH. They are 18' wide and over 55' long and flare out very wide where they meet the road. Some thing that make backing difficult is other vehicles at adjacent sites or across the road that are up close to the road.
My votes would be for 600 or 700 since they are long-and-mostly-straight loops with sites angled off to the sides. The circular lollipop loops can get tight if your in a curve (the loop roads are one-way so going around one means curving to the left) and have to back into a site on the left (inside of the loop) rather than on the exterior/outside of the loop. I call them lollipop because the loop entrance is at the bottom of the stick and the sites are spread around the circle inside and outside. Loop 1000 is also not lollipop (just like 600 and 700 aren't) but has a good straight section in the middle.
If I had to choose a lollipop loop, it would be 1100 because the sites in it seem a little further apart so the chances of having to deal with someone's vehicle are lower.
There are apps and internet resources for the Fort that touch on the backing/parking aspect but the grades are subjective: "one person's EASY is another person's HARD" is the feedback I've heard.
Finally, if you get to your site to back in and it still looks tough, you can drive around looking for an empty Premium you like and call back to the Fort Front Desk (number on the check-in sheet) and ask to be transferred to it. They will if it's available - but be aware some folks like me use the Early Check-in option showing a 1pm check-in time so I might be assigned loop 123 for example and get my text assignment at 1pm but I don't actually arrive until 4pm. If you see it sitting empty at 2pm, they can't move you to it because I've already "checked in" early. But if it is free to assign, they will give it to you.
Tell DH not to stress.
Bama Ed
PS - now I'll wait for one of these guys with the F350 extended crew cab and 45' toyhauler to come along and chime in...
PPS - I have no idea about SeaWorld.