First! Drive the speed you are comfortable with and able to maintain safe and complete control of your rig. DO NOT OVERDRIVE you own skills, ability, or comfort zone because you feel pressured by other drivers around you. TO HE!! with them. It's about YOUR safety here.
Second! Just stay in the slow lane and let everyone pass you. Except for occasional on-coming merging traffic, everyone will zoom around you. Let them go and keep smiling. Be safe!
Third! Fuel mileage. The faster you go, the more fuel you consume.
Case in point: Towing my 41 foot fifth wheel at 50 MPH I get 12 mpg. (yea, really). At 55 I get 11 mpg. At 60 I get 9.5 - 10 mpg. At 65 I get 8.5-9.5. At 70, I get 8. I'm pulling with a 3500 diesel dualy one ton.
My sweet spot is 55 mph. But on interstates I travel at 60. It's comfortable, non threatening, safe for the tires (remember your tires are rated for speed also), and comfortable. I have good control of the vehicle, and if anything unexpected occurs (like the day we had a tire blow out on the fifth wheel), I have 100% control of the rig.
Drive your comfortable speed, stay in the slow lane, and ALWAYS put your safety first above those idiots who think the speed limit is just a suggestion. Remember, you are traveling in an RV ... not a sports car!