I’ll be honest with you. In 6 weeks you will very likely lose size in 6 weeks on such a trip. I’ve had 2 friends do something similar and they were both EXCELLENT at body weight exercises (so they could do a wide variety of them during the trip0 and they all lose a pretty good amount of muscle.
First because of the food. In Asia protein portions are very low, especially if you are trekking and going through villages instead of big cities (not many big cities in Mongolia). And your overall caloric intake will also likely be much lower than here.
Second because your daily energy expenditure will be very large, you will likely walk a metric ton.
Third because your sleep likely won’t be that good and that plays a big role in muscle growth.
While body weight exercises can help you maintain muscle mass to some extent. You are likely not profficient enough in them to do the more advanced variations that provide enough demands to compensate for the lack of loading.
You can try to minimize muscle loss by applying all the strategies mentionned above. The key IMHO will be to try ti get in as much protein and calories as possible. My friends did bring protein powder with them to help with that.
“Planned overtraining” for 1-2 weeks prior to leaving is a good strategy, and should prevent muscle loss for 2 weeks or so.
But might as well come at peace with the fact that you will lose muscle, otherwise you will be obsessing with it and won’t enjoy your travels. But the good things is that the muscle that was lost will return pretty rapidly when you get back to training.