There is virtually no difference between that Dometic part and the Supco SPP6- both are PTCR based start devices. Supco does not recommend the E series for RV applications- because if you shut the compressor and try to restart it right away, the E series will put the start cap in the circuit, a PTCR device will not. My response to that- doesn't short cycle the A/C (most new thermostats will not let you anyway).
DuoTherm does make a couple of better options-
p/n 3102903.006 , which is a true potential relay (unlike the Supco "E" series quasi potential relay), and a really good, though very expensive start kit, the
SmartStart- unlike anything else on the market (at ~$370, it better be :) ).
Another option I am looking at is the 5-2-1 compressor saver- a true 3 wire potential relay.
Note that the benefit for the non-PTCR type kits is really only in running off an undersized genset- they keep the start capacitor in the circuit until the compressor comes up to 75% of speed, which takes a fraction of a second longer if the genset bogs down.
