I've been meaning to share this suggestion with others...
Moving around in the dirt and grime as you work under the bus isn't much fun. If you're on a concrete pad, a crawler can help. But I've found a solution that has worked wonders for me.
I use a 4' x 8' sheet of 1/2" foam-board insulation (from Home Depot). This is the same stuff I put under my flooring. It is skinned (aluminized) on both sides and slippery as hell (almost like it has been waxed)!
I used about half a sheet or so. I'm on a concrete slab and I'm a big buy (240#). Crawling around on a concrete slab is hard on hands and knees. I throw half a sheet under the bus where i'm working (with a couple of feet sticking out), lie down on it and slide under the bus. And I mean slide!
At only 1/2" thick, it takes no room from your "head space" under the bus. It offers a bit of padding for when I kneel on it. It insulates my body from the cold ground. It's flat and doesn't bunch up like a blanket would. If the concrete is wet, it keeps me dry. But I can't say enough about how easy it is to slide around on it on my back. I can lie down on it and use my hands to grab parts of the bus above and pull myself where I want to go. It has virtually no weight so moving it around where I want is a snap!
It gets a bit beat up over time with a gouge here or a tear there. Dents from kneeling on the edge. But it keeps performing and it's cheap! I plan to bring a piece of this stuff with me on the bus for unexpected maintenance.
I do not recommend this if your on uneven ground. This works very well on concrete but if your in the dirt and it's not very flat, this stuff would likely break in chunks. But if it's flat under there give it a try!
I can't say enough about how this has made working under the bus so much easier. I hope this idea can be made to work for you!