I thought I had read during my research that the linear movement and accelerations of the vehicle could cause significant errors in the accelerometers measurements?
True. If you need to level the load dynamically, you are opening a 55 gallon drum of worms.
How large (physically) and massive is this load?
How fast will the vehicle be moving?
What is the range of slopes to be encountered?
What mechanism will be used to level the load and how is it powered and controlled?
How quickly must the system respond?