Selling RPOD 151- Fair price?

It depends.  It depends on where you are located.  You state the decals are gone, probably due to too many years from direct sunlight.  How does the rest of the exterior appear (oxidation, surface cracks or other irregularities in the gel coat or fiberglass sheet itself)?  Has it been maintained as far as keeping up on caulking and dicor around the A/C unit, FF fan, and 2" pipe vent, and spoiler on the roof?  Has the 1" white vinyl inert trim been replaced (as well as changing out the 1" screws underneath) or is it cracked?  This is an important thing to check out at not only on top but also in the front and rear of your 151. Any water damage (you do state no leaks but. . .)?  This would be apparent by stains on the interior ceiling or along the wood in the stow areas.  Is the floor solid or mushy in spots?  Still have the original Dometic RM 8501 fridge (normal lifespan is 10 years) or has it been replaced and when?  Has the 151 been "lifted" with the 3.5" lift kit or still in its original state of sitting lower to the ground? 

These are some of the more major items of interest to any potential purchaser.  Once the above questions have been addressed I would check out what similar sized trailers would sell for in your area and go from there.

What model of solar panels do you use? I have a built in plug in for solar but I don't know what type I should get.

If possible, take some time to clean up the exterior with some gel coat restorer and then a coat of wax. Also, follow Jato's advice a full examination of the rig. Make sure everything works. Folks are always looking for weekend campers and hunting trailers. It doesn't have to be perfect, but should be solid. 

When we sold our 179, we kept an eye out on Craigslist to see what similar trailers were going for. We also called RV dealerships to see what they had used in stock and what they offer in trade. Taking all of that into account we set our price. For reference two dealers including Camping World offered us $8,000 for the 179. We sold it on Craigslist for $18,500 which was about the same as other similar sized trailers

Good luck