The 13' Escapes can be made to order with bathrooms, but you do lose the front bunk, and you didn't want to have to buy new (don't think they are being made any more anyway). You mention that maybe in the future you would want to take your Mom along, how much into the future? Although buying a trailer is a big purchase, FGRVs hold their value if cared for and maybe getting what you want and need now is a good idea and if the need arises for a larger one get it then.
My husband always says if my Mom would go with us he'd sleep in a tent outside! (we need two beds, one trailer size bed doesn't work for us). She's 89, the idea intrigues her, but she is too frail for camping. We took her on a trip, stayed in a place with cabins and camp sites, she got to have her own cabin. Worked very well.
I agree with the porta potti idea, that's what we used in our first FGRV - a 13' Burro
, just slipped it under the bed. There is no privacy, a factor if your Mom is along, but then there isn't much privacy in a 13' trailer anyway.
There may not be a trailer that meets ALL your needs. Perhaps if you find one that needs a lot of restoration you could build in a small, toilet only bathroom, likely you will need to sacrifice something - a closet, a bed, a frig, to create the space.