I'm not an avid RV tow expert, in fact I just took delivery of my first RV (Hemisphere 356QBQ). I did drive a semi-truck for years (over 1 million miles) when I was younger and have had this issue with big-rigs hundreds of times... based upon my experience, what you describe is directly related to one of 2 potential issues.
1. Too much weight behind the rear axles (or possibly directly upon the rear axles).
Answer: I recommend you store most of your weight towards the front of the trailer to keep the load dispersed across all axles.
2. Sliding hitch too far back.
Answer: I recommend sliding the hitch forward until you ensure proper weight distribution across your drive / steer axles.
I'm unsure which of the two issues (or perhaps a combination of the two) you're dealing with. Best solution is to go to a truck stop with a "Cat Scale". These scales allow you to get loaded weight on every axle. The bulk of your weight should be dispersed across your vehicle and the weight on your trailer axles should never exceed the loaded weight of your pull truck. If you notice on your scale receipt your RV axle weight is are bearing the brunt of the load, this is your issue. As a general rule, your trailer axles should not bear more than 40% of the load unless you've fine tuned your ride.
If you still have an issue once you get your load right and confirm at the scales, let me know... there are a couple other scale tricks that might help but I don't want to hijack your thread.
2014 Salem Hemisphere 356QBQ
2005 F-350 King Ranch 4x4
2006 Yamaha AR230
"If I am unable to make the gods above relent, I shall move Hades
." -Virgil (The Aeneid)