You really don't need to heat your solution tank. You don't want to be
shampooing your carpet with ice cold water, and warmer is better, but as
long as your water is tepid or better, that's all you need. I have a
commercial carpet extractor that cost me about $2000, and it doesn't
have a solution tank heater.
If I were you, I would just buy a couple of insulated plastic coolers
and take warm water with you in them to job sites where the water is
Are you not getting good results with your machine and feel that the
water not being hot enough is the cause? Maybe if you can explain
exactly what the issue is, I may be able to help.