Bucket,
You should be able to get a portable 110 volt diesel fired hot box for around $1200.-1500.
I beleive Mark-one of the sponsors of this board sells a portable wheeled hot box made by Tuff that would fit your requirements.
Here is a link to his site.
www.pressurewasherzone.com
Mark is very helpful and can assist you in making a selection based on your needs.
Basicly you have a short h.p. jumper hose, maybe six feet or so that quick couples into your p.w. and than connects to the hot box.
The water exits the p.w. under pressure runs to the burner via the short jumper hose. The water is heated in the coil and runs to your gun.
The hot box will usually have a thermostat, flow switch, high temperature switch ect, fuel filter.
When you are spraying the burner will cut on and heat.
Release the trigger and the burner will cut off.
You don't want to run in bypass to long because the water in burner coil continues to absorb heat. This is because the flow has stopped.
If you want to rinse with cold water you would have to manualy switch the burner off or purchase a remote kit for it. The kits run around $1200.
You will be amazed with the differance hot water makes. You may also see that you can use a little less chems as a result.
A burner for your requirements will use approximatly 2.5 gallons of diesel fuel per hour. You generaly will make this cost up in shorter cleaning times.
Purchse a GFI jumper to plug into you generator for your safety.
Hope this gives you the insight you were lookimg for.