This is a common concern. My take is this is 3rd RV with ducted heat for me, 4 others without. The ducted ones all have less power/air flow the further away from the furnace the vent. Yes, there could be something limiting the flow in yours, but maybe not.
The thing is the RV is not nearly the sq ft as a home and I find they do OK overall inside the RV. I camp down into the teens at times. The walls/floor are always cold. Windows the coldest. Overall the air temps do okay. From the hottest to the coldest vent is what distance 10-12 feet. The ducted heat RVs are way better than the older ones without.
2013 24RKSWE (27ft TT) Cougar 1/2 ton series
2013 Ford F350 4x4 CC 6.7 engine, 8 ft bed 3.55 rear end lariat package
Retired from Oregon State Police in 2011 than worked another 9.5 years as a small town traffic cop:
As of 05-2020, I am all done with 39 years total police work. No more uniforms for me.