i don't have the smallest suburban hw heater so i am assuming that it is the 10 gal. i do not or have i ever had to run gas and elec together. we have plenty of hw. the last time i was on gas was on my initial outing in winter of 2002. that may be a different story on a 6 gal.
tankless...my niece installed one in their home and they really like it.
i would be inclined to try what u have first and see how it works out for u.
what i would think was meant by reversing the bypass is, if it is in winter mode, hw heater bypassed, put the valves in normal mode (bypass closed and water into the hw heater open) and fill the heater.
in my home, i have a hard time justifying a thankless heater. i'm all electric and the electric usage on a tankless is huge while heating the water. i also use the waste heat from my air conditioner to heat the water.
if ur going to replace anyway, explore ur options. if not, i doubt that u will pay for the replacement with the difference in usage of propane...not to mention the times u would be able to run on park electricity and not use propane.
on my last trailer, i installed one of those small 110v hw heaters from the lumber yard and provided a separate extension cord for it. located the tank between the landing gear.