When was the last time you replaced the transmission fluid and filter. Sounds like the transmission is overheating. Do you have an auxiliary transmission cooler on the unit. Did the problem just start? Have you successfully towed other things with this truck? Yo7 did not say what size 5er you have. Are you exceeding the tow capacity rating of the truck.
Happy Camping, Safe travels and may our paths cross someday!
40' 2000 Monaco Windsor, ISC350 Cummings w/Allison 3000 6 speed and Jeep Cherokee toad. We're all old but, we keep on trucking!