Without experience in accelerometers, I deduce

A= k cos T + f(t)

where A is the device output, k some constant that involves g and device gain, T is the tilt angle, and f(t) is the "noise" produced by traveling/moving in an irregular surface.

I think that passing A throu a low pass filter with adecuate cutoff frecuency, you can get the tilt angle T, but with some delay, deppending of LPF.

Daniel