I feel for you. My truck is just over 11K lbs with the camper and I sink a good 4" into the water soaked ground even before pulling out my 7K lb trailer. Last time I had to put the truck in 4wd just to get to the trailer to prevent spinning out even deeper grooves/ruts. I was able to use the tongue weight to my advantage when pulling out the trailer, but I did find some spots gave an additional 2" with the the extra weight (all this was without wheel spin and just ground compaction).
I'm in a similar situation where I do not own the property (keep trailer at a friend's location). Even though I have offered to gravel in this section for better drainage and traction at my expense, he does not want rock in that area. Believe it or not, the ruts are gone within a week of rain and the ground is again level.
I agree with the others that pavers or gravel would be the best solution to your problem, but you may need to stay with temporary solutions if the owner does not want to alter his property. Have you considered using an old carpet for traction? It would still compress and rut underneath, but it would give you traction. The carpet could be rolled up and stored on the MDT or left at home once the fifth wheel is extracted from storage.
Any chance of relocating to another spot that is dryer or has more turnout space?