Thinking a bit more about this and looking at some of the available info on lead acid EVs, it looks like weight is the big drawback. Also, the higher voltage systems (and 300 volts is really high for an EV) get much better performance but the battery life, even with deep cycle batteries goes way down. The most successful conversions seem to be small pickups where you can add a hefty 100+ volt system and not affect the overall weight and aerodynamics that much.

One interesting alternative on the trailer idea: One concept proposed was a pure EV with a 50 to 75 mile range and an optional trailer that would have a diesel generator and power cord for longer range trips.