ROYBUCK.....DO you have a SmartWheel or is your cruise control on the side arm support. I hate the feel of my throttle and I do 90% of my driving with the SmartWheel's cruise control buttons. Every push on the SmartWheel cruise control raises or lowers my speed one mile per hour. Even in light traffic or accelerating onto a freeway onramp, I set the cruise and then just thumb it up or down as needed.
Here's another issue. Typically, when you're driving you're truck, there is much LESS stress. It handles better, less to worry about and maneuvers without a lot of effort. When you get into the RV, your stress level goes up and you become more tense while driving. Being tense causes your muscles to tighten up more quickly.
As others said, change your seating position. Tilt the back rest farther back so you're at a different angle. Move the steering wheel to a different spot. If the throttle pedal isn't close enough, add a block to it.
I found that my brake pedal and throttle are so close to together and at the same height that I was often hanging my foot on the edge of the brake pedal. I had to look down to make sure I wasn't on the edge of the brake pedal. I mounted a small block on the center cabinet just to the right of the throttle pedal. Now when I take my foot off of the brake and move it to the throttle, I feel the side of my foot rest against the stop and I know I'm clear of the brake pedal. Little changes like this can help make your drive more comfortable and cause you to drive and not be as tense.