With a six gallon tank, you can't take a 45 min shower. Otherwise, the system should be able to self regulate and provide consistent hot water temperatures whether on shore power or using engine heat.
Adding the tempering valve will allow safely running the tank hotter, effectively giving you more usable hot water (since you're mixing more cold water with it when you use it). Keeping the tank at 140-160 would be very dangerous without a tempering valve - particularly with kids or anyone with impairment or lack of heat sensation.
@jssailem, I think
@jviss is concerned that if he adds new check valves in his system, as part of his tempering valve assembly, he may wind up with an over-pressure problem. It hasn't been too long since we worked through an over-pressure issue on a hot water system for another boater. That turned out to be caused by a check valve that the owner was unaware of.
Depending on how his system is piped and where he installs the tempering valve, he may or may not have an overpressure issue. But, it's always best to review the system schematic carefully, with your modifications drawn in as you intend to install them - before you start cutting pipe.. I'm sure he's already thought of this. Even though these systems don't get terribly large, they can have some of the same issues that larger systems have.
You know how we are: if it still works, we haven't added enough features.