Transmission will stop pulling when it get hot

Sounds like you need to find a competent transmission shop.

Check oil level add Lucas stop slip.

you can check internals by having a pressure test done on the transmission in car at an accredited transmission shop. With the modern transmissions and the new fluids used oil additives are not generally recommended as they stop the bands and clutch pack from binding up and working but increase the ability to slip and so not work properly/. Have the test done and discuss the problem with the shop experts.

If it's an automatic, I think your transmission is gone. Done. Done in.
If the engine wants to roar, but the legs won't move, well, it's the transmission. When the fluid inside it gets hot again, the gears start slipping.
That's my take on it, humphrey, but of course check the fluid level and quality or appearance and smell, you know, before you do anything. If it was real low on fluid, like over a couple of quarts or even one, you might get lucky and keep it full and not have any more problem. If the fluid is real dark, dark brown or dark red, and has a distinctly burnt smell, ...that's very not good. The fluid should be clear and red or pink in color. Have someone confirm or not if your transmission is bad. good luck.

sounds like a dirty transmission filter

I don't think the trailer pulling is the problem

I thing the trans is slipping & due for a rebuild
(automatic I assume)

check the transmission fluid may be low it may need to be serviced

You might need to get a transmission cooler. It keeps the transmission cool good for towing or pulling heavy trailers.

There are two's to check.
1st way is to check it when the reansmission is still cold, whild the engine is running and it's in park. Pull out the dip stick to the transmission which should have a red handle. Pull it out and clean it and push it back in and pull it out again, at the end of the dip stick there are markings which should say cold and hot. If your transmission is still cold do to your just started it. The oil level should at least be in the cold markings or just above. 
2nd way is the same as the first, just your transmission has been working. Say you have been driving and stop for gas and everything is hot and you check your transmission oil. The you'ld look to see if your transmission oil is up to the full mark where it say's hot. 
Note: It's better to check the fuild to the trasmission when it's hot, just becareful not to burn your self.
Check for oil level and texture of the oil. Oil should be free of bubbles and have a bright red in colour. If there are sign's of bubbles in the oil or the fuild is dark burgundy colour. it's time for a tansmission oil and filter change. Try not to do a transmission flush if you have over 80,000 miles. 
Good luck and hope this helps

with it sitting for a year the filter is probably restricting the fluid flow