Makeminemusic,
Unsolicited advice from a nobody towing an 18-foot popup

. But I have camped in the Preferred loops a lot.
300 is the tightest of the three loops in general. It is the ONLY loop that has "git back" stripes painted on the site pavement where it meets the road. See the yellow paint in this picture from Halloween 2017:
The loop road is windy, the pavement is not long enough for your two vehicles and the 5er (unless you want to be THAT GUY who tries to put the vehicles side by side and drops two wheels in the dirt), and if you get to your site on arrival day and your site is across from somebody taking liberties with the "git back" stripes, it could be tough backing in. There are pedestals, trash stands, all sorts of obstacles to navigate backing in.
100 is next tightest (down at the far end from the entrance to the loop). Some sites are long and some aren't.
200 is probably your best bet. Still close, the least tight curves of the loop roads,
It is all very site-specific. Some sites (like 125) are nearly 75 feet long paved (loved that one at Halloween2017). Another like 201 is so short I had about 8 feet behind my popup to the split rail hard fence adjacent to the Comfort Station. 110 is probably the shortest and tightest to back into in 100.
The problem I see is that most Preferred sites aren't paved long enough to leave the truck hitched to the 5er and also pull the car in front of the truck. Not unless you roll the trailer tires back onto the gravel tent pad at the back of the pavement then you might have a chance. Preferred are only 12' wide paved generally so not wide enough for two side by side (Premiums generally are).
You should make a request (add it as a note/request on your reservation) that you want a LONG paved site if possible. If you don't get one, I suggest you unhook the truck and park it up in the main Fort lot and park the daily drive vehicle at the campsite.
Now FWIW I've also stayed in 400 loop (a Premium loop right cross Big Pine Road from the 100/200 loops) and they were plenty wide and plenty close to the Fort boat dock. For a few dollars more per night, you could get more space (if you can get placed in 400 that is).
It is what it is. Flexibility is key.
Bama Ed
PS - Charging for parking in campsites is hit-and-miss. I've not heard anyone recently say they've been charged. But you'll need two car passes (one for each dashboard) and that might tip them off to charge you. Who knows, by 2020, the enforcement may be completely different.