We are in the build out phase of our 170" WB 2wd sprinter, and put in a shower. Debated and debated, but in the end you want a shower inside when it is cold/************ outside. We don't go to campsites with any kind of shower, and after a few days of outdoor activity a wet towel/wipes etc. are not going to cut it. The shower will take up about 30 deep by 24 wide on the outside, and will be slightly smaller on the inside. Currently it is just framed out, and we have to add the pan/walls. We have a 25 gallon fresh water tank. Will have a 3x step shower head. 1/2 gal/min, 1 gal and 2 gal. Higher usage in case we want to camp using hook ups somewhere. It will be very tight at that interior size, even for my 5' 8" skinny frame, but you make choices and that is where we ended up. Take the comment about carrying water into heart too. You can cube out and GVWR out a Sprinter pretty quickly. Even taking "navy" showers, you still go through a bit of water much faster than people think.
A couple things to consider. It is only wasted space if you choose not to use it for other things when not showering/toileting. I saw a vid of the Winnebago revel. They have a fiberglass shower that has multiple steps in it, and they combine it with shelves. Worth a look if you can find the vid. Obviously moving stuff back and forth between the shower, and likely the top of your sleep area is a bit of a chore, but a choice you will have none the less. Then there is the whole thing about wiping down walls every time as you don't want things to get nasty, but an option none the less. Are there other things you can put in/take out during travel, than can live outside when not driving? An inflatable SUP, some other wet type gear perhaps. Don't know your hobbies, but worth a look. Finally there is the option of stepping thru the shower to get in/out of the vehicle. Earthroamer/cruiser/bruiser/stomper or one of those companies that do high end pop top Fuso's does this. The idea is no wasted space, and it serves as a mud room so to speak if coming in from inclement weather. I think the flow might be a bit strange in a Sprinter, but again something to look at.
Having lived with the Sprinter in it's mostly configured state has us second guessing the platform for full time use since we carry a bit of recreational gear. We spend maybe a few weeks a time in it. We lived full time for 1.5 years in a 27' travel trailer, and it was plenty. The Sprinter seems small for us after about a week, and we are not yet relegated to spending as much time inside it as one normally does when the weather becomes less than idea. i.e. it seems like good space when you can augment it with adjacent external living area under an awning. When that is no longer an option, we are both coming to the conclusion that living vs. spending a week or two at a time will be too small. Then again, we are sizing down, and coming from a Moto you are sizing up.
One other thing to consider with the Sprinter platform is that it may limit where you want to go based primarily upon the departure angle. a 144" wheelbase would do much better, but putting a shower in one of those is not the way I would go due to the cube loss. The 170" wheelbase makes the internal shower possible, but the rear hangover is an issue. You can go with a 4wd which will help some. You can even have Compass van add another 2" of travel to either platform 2wd or 4wd (which sits about 4" higher than the 2wd one from the factory) if you have the funds to do it. I wouldn't bother with the 170" extended, as it is almost hopeless. I can tell you that we drag the hitch frame of our 170" non-lifted 2wd in some places that are suprisingly tame. I can't imagine Mongolia or other locations will fit the description of tame.
Last consideration, with ANY diesel rig built after 2006 (2007 for commercial trucks) is the DPF emissions issue as it relates to out of 1st world country travel. I would highly recommend registering it somewhere that you can modify it for use outside the US without emissions issues. Keep it legal in the US, but have someone who knows what they are doing de-comission it for ourside of the US. Save the parts/tune and you can always change it back upon re-entry. That is quasi legal, but the only real way to go if you want to travel anywhere vs. 1st world only. There is a company in Phoenix that has a Mexico tuner special. Basically tune the vehicle to run without the removed DPF system, do all the labor, etc. Mexico less than 100 miles away, so that might make a good gateway plan. They will reverse everything on the way back. Mesa Blackworks or something like that. Budget about $3k for that, and maybe $500 to 1k on the way back. Maybe they will even store the parts for you for a small fee too.
Whatever you do, best of luck, and enjoy the journey.
