It sounds like the first mortgage holder did not seek a deficiency judgment. So, you should not be held liable as far as that mortgage is concerned. If the second mortgage holder received any of the proceeds (in other words, if the first mortgage was fully satisfied by the proceeds of the sale and the remainder went to the second mortgage holder) then it would have had to file a deficiency judgment, but from your account, this doesn't seem to be the case. The second mortgage holder could, potentially file a separate action on the mortgage note. The statute of limitations for these actions is usually 7 years.
If the second mortgage holder wants to obtain a judgment against you, you will be notified of the court action. In this case, you should demand that the second mortgage holder produce the original note. If the second mortgage holder is unable to do so, their action against you will fail.
The judgment will act as a lien on any of your deeded property. If the park service property is filed with the county, then it would act as a lien on your interest. Lien holders usually don't foreclose on property, they simply wait until it is sold and get the proceeds form the sale.
As far as the joint tenancy, the lien would still attach to the property. If you die, your interest goes to your joint tenant, and the lien may be extinguished.
Bankruptcy usually discharges deficiency judgments. Many times, people are able to keep their residences after declaring bankruptcy, but you should really speak with a bankruptcy attorney regarding your situation. The amount of detail required to advise you on whether you would be able to keep your property is too extensive for this format.
However, if I were in your position, I would not consider bankruptcy, based solely upon this issue, until or unless the second mortgage holder files suit. They may not intend to do so and bankruptcy may not be necessary.