20 years ago when we were taking a Vanagon to East Afri8ca, I( made a set of sand or mud 'ladders'.  Went to Home Depot and bought one 8 ft length of expanded wire lath-- the see-through mesh that plasterers of old used as the base for interior walls.  Cut the 8 footer into 2 four-foot sections.  Then cut each of these lengthwise.  Assuming you had a 32 in section (made to span two 16inch stud gaps) you now have four 4-ft sections, almost 16 inches wide.  Grind or sand with a belt sander the ragged edges.  Keep them over the engine, on a layer of cardboard, with another on top, then put your big foam pad back-- you'll never know they are there.
Not getting traction: put one in front of each wheel. When you can get up on those, move forward (or back if you have to give up on forward) 3 ft or so, place the other pair under the ends of the first ones, and so on.  I've gone close to 30 this way.  But WARNING: the lath itself is sharp all over its surface, and esp. in mud, it can be hard to pull up to move forward.  SO keep a pair of work gloves handy, too.  Cheap, easy to store-- what's not to like?