Trouble retaining mass in upper body.

Ive lifted for about a year now, and although im a hard gainer, overall ive put on weight pretty well, working out each muscle group twice a week. However, the past two months ive been doing a lot of couchsurfing and riding anywhere from 20-70 miles a day, again, on my bike. My biceps/triceps/shoulders have all noticeably shrunk in that time period, and ive lost about 5 pounds, but I think some of my weight just shifted to my legs. Before weightlifting I had a tiny upper body and big legs, as I rode a good amount back then too.

I still manage to get home to at least hit every group once, and I dont think im doing too shabby on my food intake. Ive doubled my intake of fruits and rice/beans, and shoot for twice my bodyweight in protein a day, but at 300 grams its sometimes a little tough while roving.

Im wondering, if my legs are snagging all my protein/carb/calorie intake, not leaving any for my upper body on days when those muscles groups arent worked as much. I dont know if its conceivable, but im just exploring possibilities. Should I try and do a light workout on my upper body on off days? Do you think its just that I now hit each muscle group once, instead of twice a week? Maybe im just not eating quite enough? Should I try to plan not to ride the day after lifting?

Sounds to me like you're probably just burning off too many calories from the riding so that your body is doing two things -

1. Using more of your muscle for fuel, as is common with endurance athletics done for long periods of time (like a 50 mile ride)

2. You could very well not be getting in enough calories with the riding you're doing, making it hard to maintain your upper body mass, particularly if your body is cannabalizing that hard-earned muscle to fuel your longer rides.

This is where it gets tricky, in trying to find a balance that allows you to maintain the mass you'd like while still being able to enjoy riding as much as you do. What I'd suggest is to do two things - first, make sure to up the calories considerably on the days you do ANY training, maybe starting with 400-500 extra calories/day and increasing if things don't balance out more in time. Second, if you're not already doing so, anything you can do to make sure you're well-fueled during your long rides should help a bit to reduce your body from cannabailizing your lean mass to the same degree. If you can make sure you're well-fueled before riding, and possibly that you can keep yourself in that state while riding (enegy gels, Clif Shot Blocks, etc.) to make sure your body is using nutrients recently put into it vs. working on using itself for fuel, that may make a difference as well. And re-fuel your body big-time after riding as well, making sure to get in plenty of calories over the remainder of the day.

I'm sure that there are others who ride who can probably offer more advice, but these things might just help out a bit. I wouldn't worry too much about not being able to hit each body part 2x/week as being the main factor, many of us here who lift don't hit a body part more than once/week and it works well, so I think it's just the sustained longer rides that are making this a challenge.

Cut your protein and up your carbs.

300g is not only unnecessary, but it's also an inferior source of fuel for you due to your endurance training. 150-200g a day is more than enough still.

Started counting my calories, and im intaking about 3,500 to 4,000 most days. Ive started drinking lots of fruit juice, so I think im back on track. Havent noticed any loss, or gain yet, but that seems an alright sign.

I keep reading that squats and deadlifts work well for building upper body mass, but as im already taxing these muscles on a regular basis, would it be overkill to pick up these exercises? I could probably do with building lower back strength, but would it be better to target these muscles to not risk wearing out my legs? Im also curious about your opinions on whether a likely disproportionate amount of strength in my legs to my back, would render squats/deadlifts useless until I build the back strength to keep up.