Hi SSF156,
Your shoreline cord is a no. 10 AWG wire so you could change the plug to a 30 amp, 125 AC volt plug NEMA no. TT-30P if you want to. The power post you plug into at the campground, or an added one at home, would have a 30 amp breaker to a Nema TT-30R receptacle, which limits you to a combined load of 30 amps. Your breaker box, being rated for 40 amps, is big enough to handle the 30 amp service. It sounds like you were going to change that anyway. The new power centers with a built-in 3-stage power converter/charger have a 30-amp main breaker inside the camper that feeds a breaker bus with several 15-amp circuits and a 20-amp AC unit for the roof AC unit. Your smaller camper may not need many circuits; when campers evolved to have a roof AC unit, microwave, galley dedicated GFIC-protected outlet, and general appliance outlets came along, they created more branch circuits. But still, all are limited to a 30 amp main supply.
A prior owner changed the original plug from the Sunline setup to a standard home 15-amp plug. Technically, they should have had a 20-amp plug if they wanted to supply the 20-amp breaker, but a true 20-amp outlet has a prong turned. However, they may have plugged into a 20-amp breaker circuit with multiple outlets with 15-amp receptacles. I sense/speculate they may not have wanted to deal with installing a dedicated 30 amp outlet at home and may not have realized that they sell 30 amp to 15 amp adapters to do that, so they changed the plug. Most campgrounds at least have a 15 amp or 20 amp outlet at the power post, so they never cared much, but they were limited in power.
You can leave what you have if you want to; the plug rating limits it to 15 amps.
I hope this helps.
John
