First, the only "Miniature" campers I am are aware of are those like the 8" long U-HAUL
toy RV on my bookshelf. Most of use hereabouts consider FGRV's to be the "Right and proper size" for a towable RV.
About 13' FGRV's.... The terms "13'", "sleeps four", and "with bathroom" are pretty much mutually exclusive terms and I personally don't know of any such floor plans that even exist. The usual floor plan in a Boler type of FGRV calls for sleeping for two adults on the dinette bed and another two children on the front bench seat when made into bunk beds. As most of the upper bunks are limited to 100 lbs or less, it pretty much rules at least out the top bunk for others than children.
For example, in the 13' SCAMP, the optional bathroom replaces the entire front bench set and then some, leaving only sleeping room for two on the dinette/bed.
The porta-potty is a customary answer, but front bunk sleepers usually have one word for, basically living in the midnight loo, Eeew.
My personal rig is a 1973 Hunter Compact-II and it's age has never been a problem. The are 40 y.o. FGRV's in great condition, and there are 5 years old FGRV's in wretched condition. Let condition and price be factors over age.
As it can have considerable effect on choices, let us know what you will be towing with and, if known, what it's owners manual sez about maximum towing weight
and tongue weight