How about one RR tie sideways and two RR ties side by side (4 total, 2 on each side). The two tied together would sit on the one that is sideways. That will give you the height you need and is pretty cheap. If you use something like this, be sure and secure the two side by side ties together so they can't separate. You will only need to dig down to put the back end of the ties on ground level or you can slope the ties to get an angle on them so you can drive up on it. I'm doing this now and I have some gravel piled up against the ties to keep them from moving round on the ground. I've had them about 6 years and no problem so far. Only thing is, it puts the back of the m/h fairly close to the ground. I have to back down, if I need to work on anything from about the rear wheels and back.