hmm im not sure i follow you. how is rims more expensive than herms? if you were only wanting to use 120v for it i think rims would be the way to go. you need a pump for either method...you can still heat your water on a burner and boil on a burner if thats what you wanted? or if you already have an electric kettle and pump it would be an easy upgrade.
on the other hand you should be able to get your cooler to hold temperature better too. read this.....
especially this part:
"The Single Kettle/Burner Method with a simple picnic cooler MLT:"
preheating your cooler first with hot tap water then following the advice in the article you should be able to do fine. also get a can of spray foam and fill in the lid on your cooler to help it hold heat.
why do you use ro water? is your that bad?