After testing Mudsdale out in Alpha UU, he seems to be made for the role of physical tank. 100/100/85 isn't fantastic bulk, but it means you can tank a neutral hit from almost anything, even before any Stamina boosts. Speaking of which, Stamina and a ground typing makes Mudsdale very good at switching into damage, and subsequently forcing a switch.
His biggest draws are his movepool and speed tier. Mudsdale has no reliable recovery outside of Rest, and can't use SR (yet, might change when BP move tutors return). He can phaze with Roar though, and has access to a pseudo edgequake with Rock Slide over Stone Edge. Having 35 base speed may not sound like much of a difference at that low a tier, but it does matter when facing other slow attackers. Golisopod and Araquanid both outspeed Mudsdale, and both can easily blow him away with water STAB and high attacking stats.
I've used a couple Mudsdale sets, and I have to say that an offensive Restalk/RestoChesto set works best for me.
Mudsdale @ Leftovers/Chesto Berry
252 HP / 252 Atk / 4 Sp.Def
Adamant
-Earthquake
-Rock Slide
-Rest
-Sleep Talk/Toxic/Roar/Close Combat
The main purpose of this set is to switch into a physical attack, and do some damage to a switch-in. I didn't make any specific calcs for the EVs, so feel free to say any particular thresholds that you'd want to counter certain threats. I decided to go for damage over defense, as with +1 Mudsdale already rivals the bulk of max defense Donphan. Restalk gives me a way to recover after switching in and getting 1-2 Stamina boosts, letting me keep him healthy for future use or to go and do as much damage as I can. If you're going for RestoChesto, then you can replace Sleep Talk with either a utility or coverage move.
In a team, Mudsdale loves nothing more than health support. Z-Parting Shot is particularly good, as it lets him get full health and gets to eat a -1 attack for a free stamina boost. In return, Mudsdale works best in doing what ground types do best: switching into rock and electric attacks. He's pretty much Donphan in that way, but sacrifices utility for longevity and hitting power.
Overall, I was quite surprised at how effective Mudsdale is at his role. If it wasn't for his ability letting him blanket check physical attackers, I wouldn't be surprised of he ended up in RU instead. In the future, I could see Mudsdale getting better with access to Stealth Rock. The best way to send him to OU is to make him Hippowdon 2.0, and give Mudsdale a recovery option outside of Rest.