I have always jacked up my RV by placing a bottle jack under the spring mount on the axle. However some axles are not as strong as others, and trailer frames can be particularly weak due to manufactures who spec really thin metal to keep the weight (well cost) down to a minimum.
I have heard of frame damage on some frames. You have to be careful not to put to much weight on a single point of the frame while lifting it. That said, I would consider it safer to have more than lifting point. So you might jack up the unit by the frame a bit, and support that with jack stands or perhaps a stack of 4X4.
While supporting the frame, then jack up a axles one at a time, and then put the tires on blocks, then put the boards in that you wanted to.
I was also camped on sand, and that area was subject to flooding (it was like a creek running under my RV at times) and my tires sunk into the sand, as well as the 6" diameter bigfoot jacks. I had to raise the jacks, then put more dirt under the jacks and 30" long 2X10"s. Then I could lift the whole RV (with the jacks) and then put more dirt under the tires. I was lucky that my Ford frame is strong enough to not bend at all!
Good luck,
Fred.
