Could be a few things...
First, as I’m sure you can guess, the garage is not nearly as well insulated as the other portions of the rig... particularly underneath the floor. In fact, there’s NO insulation under it.
Next... you only have that one furnace vent into the garage, and as you said, it’s a long run. If you don’t feel a significant amount of air coming from it when the furnace is on... pull the vent from the wall and verify the hose is fully connected to it. Also if you don’t feel a significant amount of air coming through the hose... check for tears and/or leaks in it that may be allowing your heat to hemorrhage into other areas.
And finally... the only sensor for turning the furnace on & off according to temperature is at the thermostat, which is of course, not in the garage.
Now... if you don’t mind a suggestion:
Unless you’re boondocking... do yourself a favor, and buy a couple of thermostat-controlled ceramic disc heaters like this:
First off, they’re completely safe... including tip-over protection that shuts them off.
Second, you can place them where needed, and control your your temp as desired.
Next, they run on electricity... not propane. Since you’re already getting free electricity with your campsite rate... heat your rig using THEIR electricity instead of YOUR propane that you have to pay extra for. Even if you’re paying a monthly rate, where you pay your electric bill separately... these things are still cheaper & more efficient to use in the long run.
They come in various sizes & brands... but you can easily find them at most Walmart’s or hardware stores... or have them shipped direct to you from Amazon.
I’ve been using these for the past 6 years... and in all that time, the only time I’ve even turned the furnace on is to verify it still works every 6 months, or to simply take the morning chill off the rig on those mornings I didn’t set my ceramic heaters up the night before.