Our water pump must have a hose which is 20' to the water heater; and then about 20' back to the engine. The water heater probably about a foot or so higher than the engine' pump (which is where the impeller is on most engines, I believe).

If the system is "PASSIVE" (except for a fan when you turn it on, or control it with a thermostat) you're just routing it through one simple copper straight section, from a series of fin tubes take's off some of the heat, then it's probably not that big a deal.

If you are trying to run it through a branch supplemental loop, or through a series of radiators which have more complicated pathways, I'd be a bit concerned with it the affect on flow;

Moreover, a completely passive flow would leave a pretty hot radiator there, regardless of whether you wanted heat and turned the fan on. (I woudn't want the cabin any more heated in 90-100 degree F weather, myself.) You might consider a supplemental pump which is powered off the batteries, which presumablly will be supplied by the alternator when the engine is running.

In that case, you need a pump rated for very hot water; however, it could be on a loop that is powered on when you need it and "dormant" when you don't.

You really don't want the heat in the summer -- as they say, so you might need a solenoid valves to power the "shunt" and return it to the hotwater line.

There systems made that do this with, or without, little boilers.